MOST COLOR WHEELS you find at art stores, or images of wheels you find with Internet searches aren't too helpful for digital photography. While they may illustrate the visual order of the colors, they aren't too helpful if you want to mix colors digitally. They may even be quite misleading. So I created my own color wheel using the primary colors found in the sRGB standard, which is used by digital cameras, computers, and high-definition television.
This color wheel shows the correct relationships between the red, green, and blue colors that are primary in the sRGB color system, as well as their opponent or secondary colors of cyan, magenta, and yellow.
These primary and secondary colors are the brightest and most saturated colors that can be generated from the sRGB color system. The coding in each color circle gives you the formula for generating the color: for example, cyan is GB, which means that Red=0, while Green and Blue = 255. Halfway in between the primaries and secondaries are bright tertiary colors. These tertiaries are coded with lower-case letters indicating half a given color: for example sky blue is coded gB, meaning Red=0, Green=128 and Blue= 255.
Some old color wheels use red, yellow, and blue as primary colors; others use green, purple, and orange as primaries. This is misleading for computer use since they don't give us a good idea of opponent colors. In this color wheel, if you mix together equal portions of colors opposite to one another, you will get a middle gray color; mixing together blue and yellow gives you a gray where the red, green, and blue values all equal 128.
If your images have a color cast, you can achieve white balance by moving towards the opposite color. An image that is too yellow needs more blue, an image that is too green needs more magenta.
UPDATE: My use of a value of 128 for the tertiary colors is not correct, since 128 is NOT the middle tone. It is for this reason that the wheel does not appear to be visually uniform: the tertiaries appear to be somewhat dark. Updated wheel can be found here.