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Additive vs. Subtractive Color Models

August 10, 2018

To understand how color management works, you need a basic knowledge of the additive and subtractive systems of color reproduction. Both use a small number of primary colors that combine to produce a large number – or gamut – of colors… but the way they do that is quite different.

In our Color Perception Part 1: The Effect of Light  post, we explained how the visible color spectrum (we know it as the rainbow) encompasses light wavelengths from approximately 380 to 720 nm. By breaking the visible spectrum into its most dominant regions of red, green, and blue, the human eye can mix these colors to create a spectrum of color.

Spectrum of Color

This is the basis behind the additive and subtractive color models, our topic for today.

RGB—Additive Primaries

By mixing red, green and blue (the additive primaries) in different combinations and at varying levels of intensity, we can simulate the full range of colors in nature. If the reflected light contains a mix of pure red, green, and blue light, the eye perceives white. When no light is present, the eye perceives black.
RGB Additive Primaries, Additive Color Mixing, Subtractive Color Mixing, Additive and Subtractive Color Models

Combining two pure additive primaries produces a subtractive primary. The subtractive primaries of cyan, magenta, and yellow are the opposing colors to red, green, and blue.

Televisions, mobile phones, tablets and computer monitors use the additive color system because they are emissive devices. They start with darkness and add red, green, and blue light to create the spectrum of colors.

The color rendering methods used by these devices are based directly on our response to stimuli of red, green, and blue light. Like the human eye, these devices must also process a large amount of color information at once—on screen. In logical fashion, these devices imitate the eye’s response to the additive primaries to create a colorful illusion.

For example, a computer monitor blends varying intensities of red, green, and blue light at each of its tiny pixels. These pixels are so small and tightly packed that the eye’s RGB response is “fooled” into the perception of many different colors when really there are only three.

Here’s a cool trick:  Put a magnifying glass up to your computer monitor and TV and watch what happens!*

CMY and CMYK—Subtractive Primaries

Printers, on the other hand, render colors on paper and other substrates, so they must work with reflected light. To do this, they employ the opposing subtractive primaries of cyan, magenta, and yellow.

In the visible spectrum, cyan is directly opposed to red; magenta is the opposite of green; and yellow is the opposite of blue. When cyan, magenta, and yellow pigments are laid upon a white, reflective substrate, each completely absorbs – or subtracts – its opposing counterpart from the white light. Printing processes use cyan, magenta, and yellow inks to control the amount of red, green, and blue light that is reflected from white paper.

When two subtractive primaries overlap, an additive primary is produced.

subtractive primaries remove additive counterpart to produce appearance of color, RGB Additive Primaries, Additive Color Mixing, Subtractive Color Mixing, Additive and Subtractive Color Models

This diagram shows how the subtractive primaries remove their additive counterpart from light to produce the appearance of a color.

In subtractive color printing, a fourth color, black (K, which stands for key) is added to make four-color printing (CMYK). If we only used cyan, magenta and yellow to make black, we would get a brownish color due to impurities in those ink colors. The black ink helps neutralize images and graphics and adds density to the shadows.

Of course, this is just a basic introduction to color models. If you’d like to learn more, check out our Color Services’ training courses, especially Intro to Color Management. 

CMYK and beyond

Interested in learning how printers overcome the imperfections of printing with CMYK? Watch for our upcoming Extended Gamut Printing post, we’re we’ll explore ways to print those colors that are too difficult to achieve using just CMYK.

*In case you don’t have a magnifying glass handy, I’ll tell you what happens. If you put one up against your RGB device, you’ll see how much red, green, and blue make up the colors you see on screen.

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