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Photoshop has many tools and techniques that allow you to work with color. In order to get the most out of these tools, however, it's important to understand how Photoshop deals with color.

The two color modes that most of us are familiar with are RGB and CMYK. While we can produce the same colors (for the most part) in both modes we need to understand that color are made in very different ways in these two models. The RGB model produces color by the addition of colors but CMYK produces color by subtracting colors.

This difference really comes from the nature of the medium producing the color. On a monitor color is projected or emitted. On paper, color is reflected.

If we look at a color wheel we can see both models.

If we look closely we notice that there is a relationship between the RGB and CMYK models. Red is opposite to Cyan, Green to Magenta, and Blue to Yellow. Why is this important?

When you open a photo in Photoshop and see a color cast, knowing this relationship will help you. For example, if the color cast is yellow then moving the photo more towards blue will remove that yellow cast. Photoshop helps you remember the relationship. Take a look at the info palette and notice how the RGB and CMYK colors are arranged.

The Color Balance dialog is obvious about this relationship but it is hidden away in every tool that deals with color in Photoshop.

The next time you are faced with a color cast problem remember the Color Wheel and how these two different methods of generating color are intimately related. It will make your color correction tasks much easier.