A color mode defines the color space within which you work. This might get a tad confusing; color modes, models and spaces are not the same thing, but rather they are interdependent. Color modes are usually offered in bitmap editors such as Photoshop, but not vector drawing applications such as Xara, due to the fundamental difference between pixels and vectors. Color modes include RGB, grayscale, CMYK (for printing), Indexed, and 1 bit/pixel, often called Bitmap mode (frequently used in faxing and a really lousy mode for creating bitmap artwork).

Indexed color mode is sort of interesting—it dates back to early color computer graphics and Compuserve’s GIF file format. In contrast to RGB color mode, Indexed color images use color “tags”, also called a lookup table instead of color component channels. There are up to 256 color tags in the header of an Indexed color image; when you open such an image, the application reads the header index and then assigns each pixel the appropriate color. This Indexed structure allows a file to be opened very quickly online, hence its popularity, but an Indexed image looks pathetic when compared to its RGB original because you’re going from 16.7 million colors to 256.

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