The use of colour
Colour publishing on the WWW is cheap, it doesn't cost extra as it does for publishing on paper. In that sense there is no reason to limit yourself to black and white or grey tones. For notes on how HTML uses colour, please look at "the secret of colour" section of this short course.

Colour and images
Colour raster images can be stored in different ways, for the Web RGB and INDEXED are applicable. RGB is also referred at as full-colour or true-colour and uses at least 8 bits per value (giving millions of colours). Images that use 256 colours or less are called indexed. As explained before in this chapter and in "the secret of colour" images make use of a colour lookup table (CLUT). The table is specially used for images compressed in the GIF system.
When a full-colour image is converted into an indexed image, this does not mean that automatically the "Websave Colour palette" is used. During convertion the colours used in the image are "mapped". The system simply starts with the first colour used, goes to the second and so on to a maximum of 256. This number includes 40 colours used as standard by the system itself.This is a left-over from the time that systems could not show more than 256 colours at a time.

All the "red " colours (128 in total) used in the image above, image size 3,18 Kb

For coloured images to be published on the Web, the designer is advised to use colours chosen from the "Websave Colour palette". This gives the designer full control of how the "other side" will see them. It also gives a wide range of different colours. When the "Websafe colour palette" does not have the colours available needed for correct display of the image, then use a palette with the minimum colours needed for a correct display.

Minimized palette only using 16 colours, image size 1,41 Kb

The reason for minimization of the palette is twofold: first of all it reduces the file size and secondly it can avoid dithering (dithering will be discussed afterwards). Remember: to display an image on the (older) computer systems only 256 different colours at a time are available. When using an image like the first one, already 128 of the total 256 are used. For practical purposes, as explained above, this reduces to 216. For all other images only 88 different colours are left and a few images more will consume them all. As soon as all colours are used, dithering will start.