What are the primary colors of light, paint, and printing?

Black, red, green, blue, yellow, magenta, cyan and white (image by Duncan Hull - CC-BY)

Black, red, green, blue, yellow, magenta, cyan and white (image by Duncan Hull - CC-BY)

Primary colors are the colors we mix to produce all the other colors. Because there are different ways to do the mixing, we need different sets of primary colors.

The primary colors of light are needed when we are mixing different colored light together. This is done in a computer monitor (whether using LCD technology, or the older CRT technology). The computer screen can emit only three colors, and every color we see on the screen is produced from a mixture of those three primary colors.

The human eye contains four kinds of receptors. There are the “rods”, which detect brightness but not color. There are also three kinds of “cones”, which are sensitive to red, green and blue light. All other colors trigger two or more kinds of cones. For example, orange and yellow trigger the red and green cones (in different proportions), while magenta triggers the red and blue cones.

A computer monitor comprises pixels (“dots”) of different colors. By using the same three colors as the receptors in our eyes, the monitor can produce any color.

Actually, there is a small range of colors that can’t quite be produced by a computer monitor, mostly because the cones in our eyes don’t have a perfect response at a single frequency, and there’s even a tiny amount of overlap between them. So the computer monitor doesn’t trigger an identical response in our brain, and the colors on the monitor don’t look quite perfect. Nevertheless, using the primary colors of light – red, green and blue – works well enough for most purposes. The abbreviation RGB is often used to refer to red, green and blue.

You might also see the primary colors of light being mixed in live theater or on the dance floor, where different colored spotlights have their beams mixed on the stage or floor.

Painting, on the other hand, works subtractively, in contrast to the computer monitor which starts black and has colored light added to it (the additive process). An artist’s canvas starts off being white (which means it reflects all colors of the light which lands on it). The artist adds paints which block the reflection of certain colors.

If the artist uses yellow paint it blocks the blue light, leaving just the red and green components (which our eye sees as yellow). If the artist uses cyan paint (a “greenish-blue”) it blocks the red light, leaving just the green and blue components (which our eye sees as cyan). If the artist uses magenta paint (a “pinkish-purple”) it blocks the green light, leaving just the red and blue components (which our eyes see as magenta).

The artist can mix the paints. Using yellow and cyan paints blocks the blue and red, leaving just the green component. So our eye sees that yellow and cyan paints mixed together look green.

Traditionally for painting we refer to the primary colors as red, yellow and blue. These are near enough for many purposes, because the artist can tweak the mix to get the desired color. Red and blue are close enough to magenta and cyan that we can consider red, yellow and blue to be the primary colors of paint. It’s just that they can’t reproduce quite such a wide range of colors as magenta, yellow and cyan. The abbreviation CMY is often used for cyan, magenta and yellow.

Printing onto paper is also a subtractive process. Here we won’t be able to tweak the results “by eye”, so we usually need to use inks that are truly magenta, yellow and cyan.

Any artist will know that when you mix the three primary colors together you don’t usually get a pure black, instead you get a murky dark brown or deep purple. It’s just not practical to get the perfect pigments and to mix them together in the perfect ratios. To get the best result, the printer will add a fourth color of ink – black – to ensure that deep blacks really do look black rather than brownish or purplish.

The abbreviation CMYK is often used to refer to the printing “process colors” of Cyan, Magenta, Yellow and blacK.

So there you have it. The primary colors of light are red, green and blue. The primary colors of paint are magenta, yellow and cyan (or conventionally red, yellow and blue). The primary colors of printing are magenta, yellow and cyan augmented by black.

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