Indexed colour
From JumbaWiki
Indexed colour is a type of color space for digital images. Whereas a normal RGB image specifies a red, green, and blue value separately for each pixel in the image, an indexed colour image maintains a table that defines a number of predefined colours, and each pixel references a colour in that table.
Indexed color can greatly reduce file sizes for images - especially large ones - using only a few different colours (because each pixel need only use a few bits to reference a space in the colour table, rather than many bits to reference subtle hues), but often results in very large file sizes for photographs or images with many subtle colour shades. If an indexed colour image has too small of a colour table, gradients and other shadings can appear blocky (although dithering can be used in some cases to reduce this effect).
GIF images - which are very common on the Internet - use indexed color, as do some but not all PNG and TIFF images.
