Why are the primary colors for light (RGB) different from the primary colors for paint (RYB)?

The traditional paint primaries—red, yellow, blue (RYB)—are a historical, practical set for artists but are not the optimal subtractive primaries for reproducing the broadest range of colors with filters or modern inks. The more effective subtractive primaries are cyan, magenta, and yellow (CMY), which directly absorb one primary light color each. This CMY system is used in color printing because it provides a wider gamut of colors than RYB when mixing.

If I mix red and green light, I get yellow. But mixing red and green paint makes brown. Why?

This highlights the core difference between additive and subtractive mixing. Red light plus green light adds wavelengths, stimulating the red and green cones in your eye approximately equally, which your brain perceives as yellow. Red paint appears red because it absorbs (subtracts) green and blue light, reflecting mostly red. Green paint absorbs red and blue. Mixing them means the mixture absorbs both green AND blue (from the red paint) and red AND blue (from the green paint), leaving very little light of any color to reflect—resulting in a dark, muddy brown, not a bright yellow.

In the simulator, mixing all three subtractive colors (CMY) makes black. Why does my printer also have a black (K) ink cartridge?

The simulator uses ideal, perfectly absorbing filters. Real-world cyan, magenta, and yellow inks are not perfect; they absorb some of their intended color wavelengths incompletely and also absorb a little of other colors. Mixing all three typically produces a dark, muddy brown rather than a true black. Using a dedicated black ink (the 'K' in CMYK) provides a pure, deep black for text and shadows, saves on colored ink, and improves print quality and cost-effectiveness.

What is the 'white' reference in both mixing models?

In additive mixing, white is the result when all three primary light colors are combined at full intensity. It represents the presence of a broad spectrum of wavelengths. In subtractive mixing, white is the starting condition—it is the color of the illuminating light source (e.g., white paper or a white screen) from which colors are subtracted by filters or pigments. If no wavelengths are subtracted, all light is reflected or transmitted, and you see white.

Color Mixing

Color mixing explores how different colors combine to produce new ones, governed by two distinct principles: additive and subtractive mixing. Additive mixing, used in screens and stage lighting, combines light sources of different colors. This simulator models additive mixing using the three primary colors of light: red, green, and blue (RGB). When these colored lights overlap, they add their wavelengths to the eye's perception. The resulting color follows the rules of tristimulus values, where red + green = yellow, red + blue = magenta, and green + blue = cyan. Combining all three primary lights at full intensity produces white light, demonstrating that white light is a mixture of many wavelengths. Subtractive mixing, used in printing and painting, involves mixing pigments or filters that absorb (subtract) specific wavelengths from white light. The simulator models this using the secondary colors of light—cyan, magenta, and yellow (CMY)—which act as primary pigments. A cyan filter absorbs red light, magenta absorbs green, and yellow absorbs blue. Overlapping filters subtract more wavelengths; for instance, cyan (absorbs red) plus magenta (absorbs green) allows only blue light to pass through. Combining all three subtractive primaries ideally absorbs all light, resulting in black. The simulator simplifies real-world physics by using ideal, pure spectral colors and perfectly absorbing filters, ignoring complexities like the non-linear response of displays, metamerism (different spectral combinations appearing the same color), and the imperfect absorption of real inks which require a black (K) component in CMYK printing. By interacting with the controls, students learn the fundamental difference between mixing light and mixing pigments, reinforce the concept of white light as a composite, and understand how color perception is tied to the wavelengths of light reaching the eye.

Who it's for: Middle and high school students in introductory physics or art classes learning the science of light and color, as well as educators demonstrating the core principles of additive and subtractive color systems.

Key terms

Additive Color Mixing
Subtractive Color Mixing
RGB Color Model
CMY Color Model
Primary Colors
Secondary Colors
Light Spectrum
Color Perception

How it works

Additive mixing applies to emissive displays: red, green, and blue light add to make the spectrum you see. Subtractive mixing applies to pigments and printing: cyan, magenta, and yellow inks each remove part of the reflected spectrum from white paper; the simple model R=255(1−C), G=255(1−M), B=255(1−Y) matches ideal filters on white light.

Key equations

Additive: color = (R, G, B) light levels combined

Subtractive (simple): R = 255(1−C), G = 255(1−M), B = 255(1−Y)

Frequently asked questions

Why are the primary colors for light (RGB) different from the primary colors for paint (RYB)?: The traditional paint primaries—red, yellow, blue (RYB)—are a historical, practical set for artists but are not the optimal subtractive primaries for reproducing the broadest range of colors with filters or modern inks. The more effective subtractive primaries are cyan, magenta, and yellow (CMY), which directly absorb one primary light color each. This CMY system is used in color printing because it provides a wider gamut of colors than RYB when mixing.
If I mix red and green light, I get yellow. But mixing red and green paint makes brown. Why?: This highlights the core difference between additive and subtractive mixing. Red light plus green light adds wavelengths, stimulating the red and green cones in your eye approximately equally, which your brain perceives as yellow. Red paint appears red because it absorbs (subtracts) green and blue light, reflecting mostly red. Green paint absorbs red and blue. Mixing them means the mixture absorbs both green AND blue (from the red paint) and red AND blue (from the green paint), leaving very little light of any color to reflect—resulting in a dark, muddy brown, not a bright yellow.
In the simulator, mixing all three subtractive colors (CMY) makes black. Why does my printer also have a black (K) ink cartridge?: The simulator uses ideal, perfectly absorbing filters. Real-world cyan, magenta, and yellow inks are not perfect; they absorb some of their intended color wavelengths incompletely and also absorb a little of other colors. Mixing all three typically produces a dark, muddy brown rather than a true black. Using a dedicated black ink (the 'K' in CMYK) provides a pure, deep black for text and shadows, saves on colored ink, and improves print quality and cost-effectiveness.
What is the 'white' reference in both mixing models?: In additive mixing, white is the result when all three primary light colors are combined at full intensity. It represents the presence of a broad spectrum of wavelengths. In subtractive mixing, white is the starting condition—it is the color of the illuminating light source (e.g., white paper or a white screen) from which colors are subtracted by filters or pigments. If no wavelengths are subtracted, all light is reflected or transmitted, and you see white.

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Color Mixing

How it works

Key equations

Frequently asked questions

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Birefringence (Calcite sketch)

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Color Mixing

How it works

Key equations

Frequently asked questions