Gif Images, Colour Tables

Reducing the number of colours in an image reduces the size of the colour table (gif) and therefore reduces the number of bits required to code each pixel. This gives a smaller file size and faster loading. Runs of flat colour also tend to become larger as the number of colours is reduced and this also aids file compression. Reducing the number of colours can give a noticeable decrease in image quality for colour photographs (below) but the effect can be quite acceptable for "poster"-work.

Gif, 256 colours, 56K colour table	Gif, 16 colours, 21K
Gif, 8 colours, 15.5K	Gif, 4 colours, 7.7K