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What is Index Color Mode?

a full-color image

an image with 64 colors

an image with 16 colors

ndex color is a product of the Internet revolution and is kind of a strange animal. If a graphic image has been saved in this color mode then it can contain up to 256 colors and has 8-bit depth. That may sound like a lot but, when you consider that an RGB image has 16 million colors, 256 doesn't sound like much.

The objective in web graphics is to reduce file size as much as possible while not abandoning image quality. One wants smaller images so they will download to a browser quickly.

Index color images are created from RGB images. RGB images can contain up to 16-million colors. When an image is converted to index color, the 16-million colors are converted to 256. This color reduction can create some image distortions as well as some banding (less-than-smooth gradations). Index color makes use of something called dithering. Dithering is the process of using a few colors to create the illusion of more colors (not unlike the dot patterns used in CMYK printing).

The attractive thing about index color mode is that 256 colors is the maximum the color mode can contain. It can contain less (between two and 256). With care, a designer can subtract colors from a graphic file so that it looks like a full-color image while containing far fewer colors with little quality loss.

When dealing with index color, it's important for a designer to keep in mind that, once an image's colors are reduced, those colors are gone for good. If a 16-color, index color image is converted back to full-color, RGB mode, the image file size will increase but the quality won't improve.

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