adjunct to Deke McClelland's article Fixing a Color Cast with Photoshop
The Visible-Color Spectrum Wheel
To feel comfortable working in the Variations and Hue/Saturation dialog boxes, you have to understand the composition of a little thing called the visible-color spectrum wheel. Pictured below, the wheel contains a continuous sequence of hues in the visible spectrum, the saturation of which ranges from vivid along the perimeter to drab gray at the center.
The colors along the outside of the circle match those that appear in a rainbow. But as the labels in the circle imply, the colors don't really fi t the childhood mnemonic Roy G. Biv, short for red, orange, yellow, green, blue, indigo, and violet. An absolutely equal division of colors in the rainbow tosses out orange, indigo, and violet and recruits cyan and magenta, producing Ry G. Cbm (with the last name, I suppose, pronounced seebim). Printed in large colorful type, these six even divisions just so happen to correspond to the three primary colors of light -- red, green, and blue -- alternating with the three primary pigments of print -- cyan, magenta, and yellow.
In theory, cyan ink absorbs red light and reflects the remaining primaries, which is why cyan appears a bluish green. This is also why More Red and More Cyan are treated as opposites in the Variations dialog box. Similarly, magenta ink absorbs its opposite, green; yellow ink absorbs blue.
Of course, Ry G. Cbm is just a small part of the story. The color spectrum is continuous, with countless nameable (and unnameable) colors in between. I've taken the liberty of naming secondary and tertiary colors in the wheel. Because no industry standards exist for these colors, I took my names from other sources, including art supply houses and consumer paint vendors. I offer them merely for reference, so you have a name to go with the color.
The practical benefit is that you can use this wheel to better predict a required adjustment in the Variations dialog box. For example, the color orange is located midway between red and yellow. Therefore, if you recognize that an image has an orange cast, you can remove it by clicking red's opposite, More Cyan, and then clicking yellow's opposite, More Blue.
Photoshop's other color-wheel-savvy command, Hue/Saturation, tracks colors numerically. A circle measures 360 degrees, so the Hue value places each of the six primary colors 60 degrees from its neighbors. Secondary colors appear at every other multiple of 30 degrees, with tertiary colors at odd multiples of 15 degrees. To track the difference a Hue adjustment will make, just follow along the wheel. Positive adjustments run counterclockwise; negative adjustments run clockwise. So if you enter a Hue value of 60 degrees, yellow will become green, ultramarine will become purple, indigo will become lavender, and so on. It may take a little time to make complete sense of the wheel, but once it sinks in, you'll want to rip it out of the book and paste it to your wall. Trust me, it's that useful.
- This article is the exclusive property of O'Reilly Publishing and Ilex Press, Limited. © Copyright 2006 ALL RIGHTS RESERVED. All images are the exclusive property of Deke McClelland. ©Copyright 2006 ALL RIGHTS RESERVED
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