Apple Color 1.5 - Manual (Page 33)

All these combinations can be seen in the illustration of three colored circles below. Where
any two primaries overlap, the secondary appears, and where all three overlap, white
appears.
Complementary Colors
Two colors that appear 180 degrees opposite each other on the wheel are referred to as
complementary colors.
Adding two complementary colors of equal saturation to each other neutralizes the
saturation, resulting in a grayscale tone. This can be seen in the two overlapping color
wheels in the illustration below. Where red and cyan precisely overlap, both colors become
neutralized.
Understanding the relationship of colors to their complementaries is essential in learning
how to eliminate or introduce color casts in an image using the Color Primary or Secondary
color correction controls. For example, to eliminate a bluish cast in the highlights of
unbalanced daylight, you add a bit of orange to bring all the colors to a more neutral
state. This is covered in more detail in
The Primary In Room .
33 Chapter 1 Color Correction Basics