Why is Three an Important Number in Color?
We see color the way we do because there are three types of light sensitive cells (called photoreceptors) in our eyes that produce our perceptions of light in the daytime (there is a fourth type that works at night when there is very little light to see). These cells are called cones because some of the scientists who first saw them thought they had shapes very much like the shape of an ice cream cone. The three types of cones each respond to different types of light and can be thought of as roughly sensitive to red, green, and blue light. Different colors have different amounts of red, green, and blue light coming from them and our three types of cones can help us figure out how much of each and therefore see beautiful colors. It is the fact that there are three types of cones that makes the number three so important in color.
Those three cones result in color perceptions that can be described with three types of descriptions. These are called lightness (how light or dark a color is), chroma (how different a color is from white, gray, or black) and hue (the color names we give objects like red, green, yellow, and blue). A bright red sports car might have a medium lightness, a very high chroma, and a red hue.
Also because of the three types of cones, we can mix colored lights or materials together to make many other colors. It just takes three distinct colors (or primaries) in the mixtures to make a very wide variety of colors. As you can see, three is an important number in color for many reasons.
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Updated: Apr. 5, 2010