Did you know that you can mix red and green to make yellow? That is far removed from the traditional mixing of paints, but it is true. The difference has to do with the two models of color: additive (light) and subtractive (pigment).
The Traditional Color Wheel
In grade school, most children are taught the traditional color wheel, with red, yellow and blue as the primary colors. The primary colors combine to make the secondary colors: orange, green and violet. A third level of mixing creates the tertiary colors: red–orange, yellow–orange, yellow–green, blue-green, blue–violet and red–violet. The order of the primary and secondary colors around the wheel is denoted by the mnemonic name, Roy G. Biv, with each letter in the name standing for the name of a color, with indigo added between blue and violet. The colors opposite each other on the wheel are complementary to each other. The high contrast of complementary colors is often considered attractive, and complementary colors are the source of many optical illusions.
The traditional color wheel is still taught in school and used by some artists for a very simple reason: it works pretty well. Combine yellow and blue paint, and you really will get green. The colors also seem to match up fairly well with the colors of the rainbow and the colors of light produced by a prism.
But it is not that simple.
Additive Colors: The Primary Colors of Light
If you flick a water droplet onto a television screen (or use a magnifying glass), you can see that the screen contains only red, green and blue pixels. When these tiny dots are viewed from a distance, they combine to make all the other colors. These are the primary colors of light. Just as in the traditional color wheel, the primary colors combine to make secondary colors. Red and blue combine to make magenta, blue and green combine to make cyan, and red and green combine to make yellow. This may seem strange. We are used to thinking of yellow as an important color, but where did cyan and magenta come from? On the other hand, it may be easy to imagine blue and green mixing to make cyan (which is sort of like blue-green), or red and blue mixing to make magenta (which is sort of like violet), but how on earth could red and green combine to make yellow? And yet that is exactly what happens, as you can see in this short demonstration:
In this simple video, you can see that where the red light is blocked by the golf ball, the shadow is cyan, because only the blue and green lights are combined. Where the green light is blocked, the shadow is magenta, because only the red and blue combine. And where the blue light is blocked, the red and green lights combine to make yellow. At the front of the golf ball, where all three lights combine, the ball appears white.
Subtractive Colors: The Primary Colors of Pigment
In the video above, we start with no light, in a dark room. Then we add light, one color at a time: red, green and blue. The other way of thinking about primary colors is to start with white light hitting a white surface. White includes all colors, so to create any other color, you must subtract, by absorbing some other colors. Objects appear to be different colors because of the colors of light they reflect. A white shirt reflects all colors of light back to the eye, absorbing none; a yellow shirt absorbs all colors except yellow, reflecting only yellow back to the eye; and a black shirt absorbs all colors of light, reflecting none. Light also brings heat, which is why wearing a white shirt in the summer will keep you cooler than wearing a black shirt: the black shirt absorbs more light and heat than the white shirt.
The traditional RYB (red-yellow-blue) color wheel is a subtractive model. However, the primary colors in the modern model are cyan, magenta and yellow. If you have a color inkjet printer with separate cartridges for each color, you may notice that the colors it uses are cyan, magenta, yellow and black. This is the CMYK model. K stands for “key.” In four-plate printing, black is often the color used for outlines, and the plate that all the other colors are keyed to.
In the subtractive model, the primary and secondary colors are the same as in the additive model, but reversed. So yellow and magenta combine to make red, magenta and cyan combine to make blue, and cyan and yellow combine to make green. This model is used by some artists combining paint colors, instead of the RYB model. Though CMY is considered more scientifically accurate, many still consider the RYB color wheel to be more aesthetically pleasing in terms of the complementary colors it suggests. Both points are valid. No one suggests replacing the traditional Christmas colors of red and green with red and cyan. However, if you stare at the center of this red circle for 30 seconds, and then look at a white page, the after-image you see is probably cyan, not green. (I have learned not to say with certainty what color anyone will see when looking at an image; see The Dress.)
A Modern Color Wheel
The additive and the modern subtractive models use the same colors, simply reversing which colors are primary and secondary. Therefore a modern color wheel can feature these colors without needing to identify them as primary or secondary, and can also feature the “tertiary” colors in between them.
This color wheel also accounts for saturation or chroma, with the colors becoming less saturated toward the middle. The biggest difference between the traditional and modern color wheels is in the relative emphasis on certain colors. The modern wheel shrinks the area between red and yellow, with orange a mere tertiary color. Treating cyan and magenta as full colors means that there are more color variations in the green-blue-violet section of the wheel.
As for why this modern color wheel does not have the same appeal for some as the traditional model, part of the reason is cultural. We are used to treating orange as its own color, but less so cyan and magenta. But these colors are common in nature. Magenta appears in flowers and sunsets, and cyan is the color of a shallow sea. Our perception of color is affected by how we name colors.
For an in-depth exploration of the different forms a color wheel may take, from a painter’s perspective, see James Gurney’s posts on the subject at Gurney Journey.