A colour look-up table (CLUT) is a mechanism used to transform a range of input colours into another range of colours. It can be a hardware device built into an imaging system or a software function built into an image processing application. The hardware colour look-up table will convert the logical colour (pseudo-colour) numbers stored in each pixel of video memory into physical colours, normally represented as RGB triplets, that can be displayed on a computer monitor. The palette is simply a block of fast RAM which is addressed by the logical colour and whose output is split into the red, green, and blue levels which drive the actual display (e.g., a CRT or cathode ray tube).
A CLUT is characterized by:
A common example would be a palette of 256 colours (e.g. VGA hardware); that is, the number of entries is 256, and thus each entry is addressed by an 8-bit pixel value. The 8 bits is known as colour depth, bit depth or bits per pixel (bpp). Each colour can be chosen from a full palette, typically with a total of 16.7 million colours; that is, the width of each entry is 24 bits, 8 bits per channel, which means combinations of 256 levels for each of the red, green, and blue components: 256 × 256 × 256 = 16,777,216 colours. Another common use case was low bit depth elements (e.g. 4bpp per element, with multiple palettes) composited into a high colour frame buffer (e.g. in the PlayStation 2).
An abstract, graphic example would be:
Changes to the palette affect the whole screen at once and can be used to produce special effects which would be much slower to produce by updating pixels.