The Neurological Layers of Color Perception

Seventh in a series of blog entries about color theory with live help from the ColorTheory (Step 7) .  First_post,  Prev_post,  Next_post

To summarize where we ended the last time, we showed the RGB wheel which represents the first layer of color processing by the eye, using the three cone cell responses. But there are several pieces of evidence that the optical system doesn’t stop there.

The opponent theory of color is based on the observation that there is considerable evidence that our eyes don’t think in terms of three quantities of red, green, and blue. Rather it thinks of three pairs of opposing colors: black vs. white, red vs. green, and blue vs. yellow.

Four Unique Colors

In analyzing the opposite colors, researchers were able to identify four unique colors of red, green, yellow, and blue. The four colors you see are the ones identified by Hurvich and Jameson where unique green is that green with the least amount of yellow or blue in it, (not the color with the most intense green).

There are several observations that support this:

For example, it has been noticed by researchers that people can classify colors along two axis, red vs green, and yellow vs blue– a reddish color never includes any green, and a blueish color never includes yellow.

Also, while the medical causes of color blindness are associated with failures in the color cones, they are described clinically as red-green color-blindness, yellow-blue color-blindness, and a monochromatic version.

The third observation concerns a phenomenon known as “optical mixing”. Optical mixing occurs when the eye sees two colors in close proximity or quick succession. If you blow up the figure, you will see that it alternates two pure colors, but the result of the eye, especially for the unique colors, can be quite different. In particular, red and green mix as grey, not as yellow as in additive color mixing.

But simply putting the unique colors opposite one another does a poor job of distributing the colors. So let us look at a more painterly approach.

The Munsell wheel was developed in 1905 (and re-notated in the 1940′s according to the current research), matching opposite colors by optically mixing along 10 opposite sides, and then distributing them equally around the wheel to be uniform in difference. It is very popular among realist painters.

The actual wheel and associated color system is actually based on color samples, not analytically on CIE XYZ. The wheel as shown is an approximation of the true proprietary system. There are other models that can be manipulated more easily and claim to be even more useful, but the Munsell wheel remains the only one that pays tribute to the ways that colors mix optically. We’ll see the latest of the other systems in the next posting.

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