Light Arithmetics: How We Perceive Color

Author: Simone Maimon

Editor: Suri Liu

Artist: Coco Zhou

People often mistake the three primary colors to be red, yellow, and blue. While these are examples of primary colors, those three do not form a complete system together. ‘Primary’ colors can combine to create every visible hue; different combinations of colors can achieve this depending on the medium you’re working with. Red, yellow, and blue are good approximations for the primary colors of physical dyes, but more accurately, the two main primary color systems are RGB (red, green, and blue) and CMY (cyan, magenta, and yellow). These correspond to additive and subtractive color mixing, respectively. But what does it mean to combine colors on a physical level?

Everything we see results from our detection of light; light is colorful because it has wave properties. Specifically, the wavelength of light, or the distance between the peaks in the waves, determines its color. For example, the approximate wavelength of red light is 700 nm, green light is 550 nm, and blue light is 400 nm. These RGB primary color wavelengths do not inherently differ from any other wavelength; they are wavelengths along the spectrum of visible light (~400-700 nm). However, they are distinct to the human eye. 

The cells in our eye detect light, sending signals about our surroundings to the brain. Rod cells detect the brightness of light and the cone cells perceive color. Each cone can absorb a small spectrum of wavelengths, detecting the light which is of the wavelength it can absorb. We have three cone types, each of which detects red, green, or blue. The cells’ information combines to illustrate a clear picture after all these messages are integrated. 

Even though we only have three types of receptors, we can differentiate between every color in the range of visible light, not just the primary three. This is because each cone can be partially activated—creating the phenomenon of color mixing—allowing us to see secondary and tertiary colors. There are two ways to create secondary color mixing: additive and subtractive. Additive mixing involves combining multiple wavelengths to perceive new colors, whereas subtractive mixing involves blocking certain wavelengths to emphasize the appearance of other colors.

Additive light mixing occurs in light-emitting objects, such as computer screens. The starting state is black, in the case of the absence of light; when the primary light colors are shone in equal amounts, it appears white. Equal mixtures of red and blue appear magenta, red and green appear yellow, and blue and green appear cyan. In a computer screen, every pixel tunes the red green and blue components to a specific intensity to alter the perceived pixel color. This results in most computer programs using the hexadecimal color classification to reference specific specific color—six-digit values in base 16 which can range from 0-F, (e.g., B321F6, a bright purple). This system can uniquely reference over 16 million colors. Every two digits—in this case B3, 21, and F6—indicate the amounts of red, green, and blue respectively. Because B3 and F3 are relatively high amounts of red and blue, this color appears purple because we perceive purple when our red and blue cones are simultaneously activated.

On the other hand, our color perception is also altered when light is absorbed or blocked in transmission. This phenomenon occurs in various physical media, which may not emit light directly, but can reflect certain wavelengths of light, and absorb others. Objects can act as an equivalent of filters, as only some wavelengths of light are transmitted. Thus, the starting color for subtractive color mixing is white, when all wavelengths are transmitted; when no wavelengths are transmitted after being filtered, the perceived color is black. The primary colors of subtractive mixing are the secondary colors of additive mixing: cyan, yellow, and magenta. When each of these filters is applied to white light, the resulting color is the color which was not incorporated to make the secondary color in additive mixing. For example, as blue and green form the secondary color cyan, a cyan filter appears red, because any blue or green wavelengths would not reach the eye. 

Ultimately, the two primary color systems RGB and CMY are the building blocks for creating any color, depending on whether you're combining light sources or filters. When our eyes detect a significantly higher amount of a certain wavelength, we perceive an object as that color. But our eyes cannot see light that has a longer or shorter wavelength than the visible light spectrum, which includes UV light, infrared light, etc., which other animals may be able to detect with their eyes. Therefore, the primary colors for humans are not applicable across species; a bee can detect 100-400 nm light, therefore, the primary colors for bees would be lower than that of humans so that they could mix to create these UV light hues. But because our cones detect red, green, and blue, light of those wavelengths can form every wavelength detectable to humans.

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Citations:

“Why Are Red, Yellow, and Blue the Primary Colors in Painting but Computer Screens Use

Red, Green, and Blue?” Science Questions with Surprising Answers,

www.wtamu.edu/~cbaird/sq/2015/01/22/why-are-red-yellow-and-blue-the-primary-

colors-in-painting-but-computer-screens-use-red-green-and-blue/ Accessed 15 Mar.

2025. 

Color Mixing and Colour Vision: Physclips - Light,

www.animations.physics.unsw.edu.au/jw/light/colour-mixing.htm.  Accessed 15 Mar.

2025.