Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

Why are red, yellow, and blue the primary colors in painting but computer screens use red, green, and blue?

Category: Biology      Published: January 22, 2015

rgb color wheel
The color system that best matches the human eye is the red-green-blue color system. For additive color systems like computer screens, the primary colors of this type of system are red, green, and blue. For subtractive color systems like inks, the primary colors of this type of system are the opposites of red, green, and blue, which are cyan, magenta, and yellow. The red-yellow-blue painting color system is effectively a corruption of the cyan-magenta-yellow system, since cyan is close to blue and magenta is close to red. Public Domain Image, source: Christopher S. Baird.

Red, yellow, and blue are not the main primary colors of painting, and in fact are not very good primary colors for any application.

First of all, you can define any colors you want to be the "primary colors" of your color system, so that other colors are obtained by mixing the primary colors. Although there may be an infinite number of color systems, they are not all equally useful, practical, or effective. For instance, I am free to create a color system where I define light blue, medium blue, and violet as my primary colors. Even though I am free to define my primary colors as such, this color system is not very useful in general because no amount of mixing of these primary colors will produce red, orange, yellow, etc. Therefore, we should make a distinction between a color system and an effective color system. The effectiveness of a color system is best measured as the number of different colors that can be created by mixing the primary colors of the system. This set of colors is called the "color gamut" of the system. A color system with a large gamut is more able to effectively represent a wide variety of images containing different colors.

The most effective color systems are those that closely match the physical workings of the human eye, since it is ultimately the human eye which experiences the color. The human eye contains a curved array of light-sensing cells shaped like little cones and rods. Colored light is detected by the cone cells. The cone cells come in three varieties: red-detecting, green-detecting, and blue-detecting. They are so named because the red cone cells mostly detect red light, the green cone cells mostly detect green light, and the blue cone cells mostly detect blue light. Note that even though a red cone cell predominantly detects the color red, it can also detect a little bit of some other colors. Therefore, even though humans do not have yellow cone cells, we can still see yellow light when it triggers a red cone cell and a green cone cell. In this way, humans have a built-in color decoding mechanism which enables us to experience millions of colors, although we only have vision cells that predominantly see red, green, and blue. It should be obvious at this point that the most effective color systems are ones that closely match the human eye, i.e. color systems that mix red, green, and blue light.

There is a slight complication because there are really two main ways to create a light beam. We can either create the light directly using light sources or we can reflect white light off of a material that absorbs certain colors. A system that creates light directly is called an "additive" color system since the colors from the different light sources add together to give the final beam of light. Examples of additive color systems are computer screens. Each image pixel of a computer screen is just a small collection of light sources emitting different colors. If you display an image of a pumpkin on your computer screen, you have not really turned on any orange-emitting light sources in the screen. Rather, you have turned on tiny red-emitting light sources as well as tiny green-emitting light sources in the screen, and the red and green light add together to make orange.

additive and subtractive color mixing
The top image shows how red, green, and blue add to make other colors, such as in computer screens. The bottom image shows how cyan, magenta, and yellow subtract to make other colors, such as in inks. Public Domain Image, source: Christopher S. Baird.

In contrast to an additive system, color systems that remove colors through absorption are called "subtractive" color systems. They are called this because the final color is achieved by starting with white light (which contains all colors) and then subtracting away certain colors, leaving other colors. Examples of subtractive color systems are paints, pigments, and inks. An orange pumpkin that you see printed in a newspaper is not necessarily created by spraying orange ink on the paper. Rather, yellow ink and magenta ink are sprayed onto the paper. The yellow ink absorbs blue light and a little green and red from the white light beam, while the magenta ink absorbs green light and a little blue and red, leaving only orange to be reflected back.

There are therefore two equally-valid methods for creating color: additive systems and subtractive systems. With this in mind, there are thus two color systems that are most effective (i.e. most able to match the human eye): (1) an additive system that creates red, green, and blue light and, (2) a subtractive system that creates red, green, and blue light.

For an additive system, light is created directly. This means that the primary colors of the most effective additive color system are simply red, green, and blue (RGB). This is why most computer screens, from iPods to televisions, contain a grid of little red-, green-, and blue-emitting light sources.

For a subtractive color system, a certain reflected color is obtained by absorbing the opposite color. Therefore, the primary colors of the most effective subtractive system are the opposites of red, green, and blue, which happen to be cyan, magenta, and yellow (CMY). This is why most printed images contain a grid of little cyan, magenta, and yellow dots of ink. Cyan is the opposite of red and is halfway between green and blue. Magenta is the opposite of green and is halfway between blue and red, and yellow is the opposite of blue and is halfway between red and green.

In summary, the most effective color systems are red-green-blue for additive color systems and cyan-magenta-yellow for subtractive color systems.

So where did the red-yellow-blue color system come from that they teach in elementary school? Typically, students first encounter color concepts when painting in an art class in grade school. Paint is a subtractive color system, and therefore the most effective primary colors for painting are cyan, magenta, and yellow. Note that high-quality paintings typically do not use just three primary colors since more vivid scenes can be achieved using dozens of primary colors. But when teaching art, it's easier to start more simply; with just three primary colors. Now, to a little grade-schooler, the words "cyan" and "magenta" don't mean much. Furthermore, to an undiscerning youngster's eye, cyan looks awfully close to blue and magenta looks awfully close to red. Therefore, cyan-magneta-yellow becomes corrupted to blue-red-yellow. Elementary art teachers either ignorantly perpetuate this less effective color model (because that's how they were taught as children), or intentionally perpetuate it (because it's just too hard to teach six-year-old's the difference between cyan and blue). Historical tradition was also a prime driver of the red-yellow-blue color system since it was historically thought to be effective before the details of human vision were understood. Since the red-yellow-blue color system is less effective, it is not really used anywhere these days except in elementary school art.

Topics: CMY, RGB, color, color mixing, color theory, light, primary color, primary colors, vision