colorindex

Color index, sometimes written as colorindex, is a numerical value used to select a color from a predefined palette or color lookup table (LUT) in digital images and graphics. In an indexed color system, each pixel stores a small integer, commonly 0–255, which points to a color in the palette. The palette itself contains the actual color definitions, typically as RGB triplets, and may also include an alpha channel in some formats.

In practice, color indexing separates image data (the indices) from color data (the palette). This allows compact image representations and can simplify rendering on hardware with limited color capabilities. Indexed color has been widely used in formats such as GIF and PNG-8, and on display hardware that uses fixed palettes, such as older VGA systems.

Color indexing trades color fidelity for efficiency. The process of choosing the palette is called color quantization, and it aims to approximate a wider range of colors with a limited set of colors. Techniques such as dithering are often employed to reduce visible banding when converting true-color images to an indexed palette.

Today, most consumer images use true color (24-bit or higher), but indexed color remains common in certain contexts. It is still used for simple icons and sprite sheets in games, embedded systems with limited memory or display capabilities, and various image compression workflows where reducing color depth is advantageous.

Terminology related to color indexing includes palette, color map, and LUT. These terms describe the collection of colors that a set of indices references and are central to how indexed images are stored and displayed.