Abstract

We live in a world full of colors. Almost every object existing in nature is assigned a color; the sky is blue; the grass is green. But what exactly blue and green are? Let’s put it another way, what is color? And how we perceive color?

Generally, color is a notion of physics and perception, and it also related to cognition. It refers to visible lights that can be perceived by human eyes. Visible lights reach to the retina and then the information is subsequently processed by the human brain, thereby generating a sensation.

Introduction

This paper explores colors in the perspectives of physics, perception, and cognition basing on researches conducted by scientists. The topic of color will be roughly divided into these six sections: attribute of color in physics and its evolution, human’s perception on account of the trichromatic theory and the opponent-color theory, utilization of color including RGB, HSB and CMYK, examples of color perception, color constancy and brightness constancy as well as color illusion with examples, and color cognitive abnormality.

1.Color Perception

1.1 Physics

In 1966, Isaac Newton discovered that white light could be disassembled and reassembled in prisms. When a narrow beam of sunlight hits the face of transparent prism, some are reflected, while others pass through the glass emerging as different-colored bands. He was the first to use the word spectrum to illustrate this phenomenon. He raised that light is made up of different particles of different colors and moves at different speeds; red light moves a bit more quickly than violet, because the angle between red light and the glass is much larger than the violet. Newton briefly categorized seven colors in the spectrum: red, orange, yellow, green, blue, indigo, and violet. From then on, chromatics evolved into a concrete science.  

In the realm of modern science, what Newton proclaimed as spectrum is a tiny portion of the electromagnetic spectrum, which refers to visible light or visible spectrum. 

In the spectrum of physics, light is a form of electromagnetic radiation that emanates from the source light, the sun and the rest of the universe, which continuously strikes the earth. Electromagnetic radiation travels in waves as wavelengths in the range from the shortest cosmic rays to the longest radio waves. While human eyes are only sensitive to a small proportion of electromagnetic radiation with wavelengths of approximately 400 to 700 nanometers. The frequency ranging roughly from 430 to 750 terahertz involves three major parts: visible light occupying the middle area, the infrared with longer wavelengths more than 700 nanometers, and the ultraviolet with shorter wavelengths less than 400 nanometers (Hoeksema, Fredrickson, Loftus & Wagenaar, 2009, p. 119). Different wavelengths target at their corresponding colors. The colors lying on this tiny visible portion of the spectrum are an approximation, because the spectrum is a continuum without clear boundaries between a specific color and the next.

The characteristics of light triggers further discussion. So how human eyes perceive these colors in the form of visible light and how human brain processes these colors?

1.2 Human Vision and Color Perception

1.2.1 Biology

Light, or electromagnetic energy, enters eyes through the image-forming system (the cornea, the pupil, and the lens) and is formed in the transduction system where the retina spread with neural receptors. The receptor cells are categorized in rods and cones, which are extremely sensitive to lights and colors. Rods are specialized for sensing lights, but they don’t perform well in color sensation; while cones are just the opposite: they are more sensitive to colors rather than lights (Hoeksema et al., 2009, p. 120). It exactly specifies the reason that human is able to perceive an object in the dark where the rods sensitive to lights are active rather than the cones that are specialized in colors. The cones are separated into three types according to their sensation toward most distinct sections in visible spectrum:the short cone (S) to blue (450nm), the medium cone (M) to green (540nm), and the long cone (L) to red (570nm). As for other area in the visible spectrum, their capability of sensation sharply decreases in succession.The RGB color model basically corresponds to the colors in red, green, and blue.

1.2.2 The Trichromatic Theory

The trichromatic theory, proposed by Thomas Young and completed by Hermann von Helmholtz, indicates that three types of cones are respectively most sensitive to a narrow region of wavelengths: the short-wavelength cone is particularly sensitive to blues, the medium-wavelength cone is particularly sensitive to greens and yellows, and the long-wavelength cone is particularly sensitive to reds. The three receptors’ sensation of color in different degrees causes the specific ratios of activity, and then the joint response determines the sensation of a unique color (Hoeksema et al., 2009, p. 127). Hence, this theory stresses that a specific color that human perceives is generated by the joint activity of three types of cones rather than separately process the information by specific receptors.

1.2.3 The Opponent-color Theory

An alternative theory called the opponent-color theory, developed by Ewald Hering in 1878, states that the visual system interprets colors in pairs: magenta and green, blue and yellow as well as white and black.Ewald Hering observed that all colors may be described as one or two of magenta, green, yellow, and blue. The phrases used to describe colors only include reddish-blue, for example, instead of a specific color. While colors like reddish-green of yellowish-blue can not be perceive; rather, a combination of red and green may seem yellow, and a mixture of yellow and blue may look white. A pair can not be perceived at the same time, therefore, we perceive one unit whenever its opponent color is out of balance, and when both pairs are out of balance, we can perceive their combination (Hoeksema et al., 2009, p. 127). It results from the excitatory and inhibitory connections between the three cone types. The information from each type of receptor processed by the brain combines gives rise to different perceptions of different wavelengths of light. Specifically, when red (L cones) and green (M cones) are simultaneously stimulated, which results in the perception of yellow (Kline, 2002, para.2). However, when blue light activates the S cone, blue will be perceived. This theory well explains the phenomenon of afterimage.Staring at green for a long time can result visual fatigue, and consequently red will come into play. The visual impact of blood can be buffered by green, which is widely applied to the interior design in hospitals.

Please stare at the picture below for one minute.

1.2.4 Modern Color Vision Theory Model

From the perspective of modern science, these two theories of color vision both make sense and they are integrated into a two-stage theory: the processing of color begins at an early stage of the receptors, and then retinal ganglion cells function as color-opponent pairs and operate on visual information after the retina. At the very level in the receptors, three types of cones specialized in different spectral sensitivities results in trichromatic color vision (Hoeksema et al., 2009, p. 122). In the opponent-process, opponent processes arise at the level of retinal ganglion cells; visual information processed is then sent to the brain through the optic nerve to the optic chiasma and enters the thalamus to synapse at the lateral geniculate nucleus (LGN). Trichromatic cone cells respond positively to one of three frequencies exhibited by photons arriving on the surface. Subsequently, the opponent cells nearby are spontaneously active, increasing their activity rate in reacting to specific range in wavelengths and decreasing it corresponding to another. In specific, when the cones retrieve red lights and green lights in equal amounts and mix them up, the cells processing blue are exhibited and less active, which results in yellow; if the retina is simulated by blue lights, the cells specialized in blue will be rapidly active and the cells processing red and green will be fired, thereby resulting in blue; if the cones are simultaneously exposed to red, green and blue lights, opponent cells nearby will be triggered tuning to luminosity.

2 Application of Color Perception

2.3.1 RGB Color Model

Addictive color is a color system that generates color by mixing three primary light colors, red, green, and blue. The mixing of two standard addictive primary colors in the same amounts can produce addictive secondary colors, yellow, magenta, and cyan; the combination of three standard addictive primary colors in equal proportion yields white light; without light, the world appears as black (Kalloniatis & Luu, 2007, para. 8). By manipulating the relative amounts of three standard addictive primary colors, the colors constituting the world can be accessible, which is so called the theory of additive color. According to the theory that the cones in human eyes are most sensitive to three types of lights, red, green, and blue, these three colors are defined as primary light colors.

Display devices, computer monitors and televisions for example, employs this theory and use these three colors’ sub-pixels in various proportions to compose myriads of colors. RGB input devices, like digital cameras and image scanners, use sensors targeting at red, blue, and green to record information of images. 

2.3.2 HSB Color Model

Color involves three traits: hue, saturation, and brightness or value. Therefore, the color mode, HSB, is alternative to HSV. Compared with of the RGB color system, HSB is more closely align with how human normally perceive color-making attributes. 

2.3.2.1 Color Wheel

A color wheel is an organization of visible spectrum around a circle by human (Markus, 2015, p. 94). However, the two ends can not smoothly match to each other. Newton discovered primary colors and illustrated a rule for mixing them though experiments on prism. He rearranged the wheel by personally adding magenta as tradition, which is so called the color wheel.

2.3.2.2 Hue

Hue is the most essential trait of color, representing the natural color of an object. It is related to wavelength in the visible spectrum. The color in different positions of a color wheel is hue. It refers to a specific color described as red, green, blue, and yellow, etc. 

2.3.2.3 Lightness 

Lightness refers to the degree of brightness of colors. In other words, it is the brightness of color lights that the rods sense in human eyes. The brighter an object is, the higher value it has. If it is in a low degree of brightness, it is dark. 

2.3.2.4 Saturation

Saturation is the intensity of a color. If a color is in high saturation, it is bright and vivid. While if is in low saturation, it is lifeless and pale. If the saturation of a photo is tuned down to zero, it turns to a black-and-white photo (grayscale image). The more complex the lights are, the less saturation the colors are.

2.3.3 CMYK Color Model

The procedure of subtractive color starts with white light, which is composed of various color lights (In order to research this topic in convenient way, the system is simplified as red, green, and blue.) When white light hits an object in white, all colors are reflected, thereby presenting a white color; while a black object absorbs all lights, hence it is perceived as black.

When white light strikes to the surface of a cyan object, all red lights are completely absorbed and lights in blue and green are reflected, generating the color, cyan. This is the same as the process of yellow and magenta perceived; magenta is the absorption of green lights and the reflection of red and blue lights; the reflection of red and green lights and the absorption of blue lights produce yellow. Red, green, and blue are the combination of equivalent amounts of cyan, magenta, and yellow. Cyan, magenta, and yellow mixed up in identical amounts creates black theoretically. 

The color we perceived on printing is the result of combination in subtractive colors, which are filtered by white lights. In the color printing, cyan, magenta and yellow are the normal primary colors (Kalloniatis & Luu, 2007, para. 9). Theoretically, the combination of different amounts of these three is able to produce a wide range of colors. However, it is basically impossible to create pure black. Therefore, a black ink key is involved as a supplementary component, resulting in the color mode of CMYK (Cambridge in Colour, 2018, para. 6). 

3. Color Cognition

The Color perception is just the first word when we talk about color cognition, as color will be further processed after cerebral cortex’s handling. In other words, color is a kind of feeling.

3.1 Color Constancy & Brightness Constancy 

In order to understand the color perception deeply, we begin by understanding the color constancy. Color constancy is an example of subjective constancy. It is a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions.

For example, when we see a red apple, our brains tends to process it as red, regardless of the wavelength and color of the light from the environment. To make it clear, look at the example: the left inner box appears darker than the box on the right—although they’re the same color. Both squares reflect the same amount of light into your eyes, but they still appear different because of the context.

But how this process works? In the process of color vision, organisms are able to distinguish objects based on the different wavelengths of light reflected, transmitted, or emitted by the object. As we mentioned above, the receptor cells to detect light are categorized in rods and cones, they send the signal to visual cortex, which processes this sensation into subjective color perception. 

Apart from color constancy, there is brightness constancy, which refers to the fact that “the perceived lightness of a particular object changes very little, even when the intensity of source, the amount of light reflected off the object changes dramatically.”(Hoeksema et al., 2009, p. 122) This perception helps our brains stay focused. 

3.1.1 Memory Color Effect & Chromatic Adaptation

There are other two terms that similar to color constancy, which are memory color effect and chromatic adaptation.

Memory color effects is defined as the hue of an object that people know subconsciously through their experience. For example, people used to think that apples are red, when we see a grey apple, our mind tends to think it is red. Researchers have found that memory color effect is cited as evidence to achieve color constancy, and to support the theory of opponent color.

Chromatic adaptation is the human visual system’s ability to adapt to illumination changes in order to preserve the colors of object’s appearance. When people adapt to one color of light and observe another color, the other color will have complementary color components that adapted to light.

3.2. Color, Optical Illusion

3.2.1 Blue and Black Dress

Is this dress “white and gold” or “blue and black”? There was an intense debate going on the Internet while people are freaked out by the realization that how they see the world is eventually a subjective construction of their brains.

To explain this, there is a word called optical illusion. The fact is that people may correct the blue to white since our brains evolved to favor consistency over accuracy.(Carmen, 2015)In order to perceive the item as the single continuous thing that it is, our brains evolved the color and shading through correction algorithms, for example, white will appear blue in dark light, but our brains still see white – it corrects the blue perception into white. As different people’s brains will make different assumptions, there was a heated discussion.

Color and optical illusion are images where surrounding colors tricks the human eye into incorrect interpretation of color. In other words, the actual color that falls upon our retina will change dramatically in different lighting conditions. So what we perceive is a constructed illusion, based upon algorithms that make reasonable assumptions about distance, shading and color – but they are assumptions, which may be wrong or misleading.

3.2.2 Checker Shadow Illusion

Other than the example we mentioned above, there is a black and white version of the dress illusion – the checkerboard illusion, which is also known as Adelson’s checker shadow illusion. The colors on A and B squares is precisely the same shade of grey, but our brains perceive them as light and dark due to the “shadow” being cast upon B area. This reveals that the actual color, which falls upon our retina will change dramatically in different lighting conditions.

3.2.3 Afterimage

Talking about color, optical illusion, we have to mention about afterimage as well. Basically, it is an image that continues to appear in one’s vision after the exposure to the original image has ceased. An afterimage induced by prior adaptation to a visual stimulus is believed to be due to bleaching of photochemical pigments or neural adaptation in the retina.  (Shimojo& Kamitani & Nishida, 2001, P2 )

Ewald Hering explained how the brain sees afterimages, in terms of three pairs of primary colors. This opponent process theory states that the human visual system interprets color information by processing signals from the cones and rod cells in an adversarial approach. It suggests that there are three opponent channels: red versus green, blue versus yellow, and black versus white. Responses to one color of an opponent channel are antagonistic to the other color. Therefore, a green image will produce a magenta afterimage as anything resulting in less green is interpreted as its paired primary color magenta. For example, If we stare at this image for 5–60 seconds and then looks at a white object, a negative afterimage will appear.

4 Color Perception Abnormality

4.1 Daltonism

Some people’s color perception is different from normal people, which will lead to different cognition of color, and the most typical example of color cognitive abnormality is Daltonism, which is also known as red and green color blindness.

According to the trichromatic theory, any color in the visible spectrum can be composed of red, yellow and blue. Normally, people can identify the three primary color, and those who have lower ability to recognize any kind of color are called hypochromatopsia, also color weakness. For color blindness sufferers, they cannot recognize one kinds of primary color, which are mainly red blindness and green blindness.

To further explain, daltonism is related to the cones in eyes’ retina. The membranous disc of the cones is embedded with visual pigments that can sense strong light and the color sense, which are continuously synthesized and supplemented by the internal segment. Most mammals including human beings have three kinds of cones: red, blue and green pigments. People who lack cones that can sense red (or green) light, are red (or green) color blindness.

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4.2 Cataract

Another example is the cataract symptom, which we can refer to Claude Monet’s ultraviolet world. Monet is one of the milestone artists in western art history, but what rarely known by people is that he gained a special ultraviolet vision at his old age due to the cataract. The world in his eyes had become blurred since 1914, but because impressionism seeks to express the real color perceived from eyes, Monet showed his abnormal color perception without reservation. 

Cataract is caused by crystalline lens’ turbid and whitish, once the crystalline lens is opaque, the vision is not only blurred, but also unable to image the blue-violet light as its short wavelength would be scattered. And that’s the reason why Monet perceived most of the color as yellow, red for a period of time.  And it was not until 1923, when Monet’s cataract became more and more serious, he had two operations to remove the crystalline lens of his right eye. The ultraviolet light began to reach his retina without obstruction and was sensed by the blue-purple cones, which brought him an unprecedented strange color perception. For that reason, Monet revised many of his artworks three years after the operation, most of his artworks were covered with a kind of solemn blue-purple color, which seems to express his pain of losing wife and child.

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Conclusion

In conclusion, color is not just a concept, it is processed and handled through retina and the cerebral cortex. According to Markus Vegh, a graphic designer from Germany, “color is related to various factors, such as physical, physiological or psychological factors, which are interrelated and inseparable.” (2015, p. 89) In terms of physical level, color is a visual effect of different wavelengths’ visible light on human beings. From a physiological point of view, our eyes would transmit the received light information to brain as a stimulus signal through nervous system, so that we have a sense for a certain color. And Psychologically, the structured way of our brains determines how we process the information, including how we perceive color.

References

• Hoeksema, N, S., Fredrickson, L, B., Loftus, R, G., & Wagenaar, A, W. (2009). Atkinson & Hilgard’s Introduction to Psychology (15th ed.).London, UK: Pat Bond.
• Kline, D. (2002).Theories of Colour Vision. Retrieved from
• https://psyc.ucalgary.ca/PACE/VA-Lab/colourperceptionweb/theories.htm
• Kalloniatis, M., & Luu. C. (June 5, 2007). Color Perception by Michael Kalloniatis and Charles Luu. Retrieved from
• https://webvision.med.utah.edu/book/part-viii-gabac-receptors/color-perception/
• Cambridge in Colour. (2018). Tutorial: Color Perception. Retrieved from
• https://www.cambridgeincolour.com/tutorials/color-perception.htm
• Carmen, F. (2015). The science behind the dress colour illusion. Retrieved from 
• https://www.theguardian.com/technology/blog/2015/feb/27/science-thedress-colour-illusion-the-dress-blue-black-gold-white
• Shimojo, S& Kamitani, Y& Nishida, S (2001). “Afterimage of perceptually filled-in surface”. Retrieved from 
• https://www.researchgate.net/publication/11811835_Afterimage_of_Perceptually_Filled-in_Surface
• Markus, W. (2015). Complete Handbook of Graphic Design. Beijing, China: Beijing Science and Technology Press.