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Truecolour (or Truecolor, Millions on a Macintosh) graphics is a method of storing image information in a computer's memory such that each pixel is represented by three or more bytes. The bits are divided into values for the Red, Green, and Blue components of the final colour, like this:

Bit   23 22 21 20 19 18 17 16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
Data   R  R  R  R  R  R  R  R  G  G  G  G  G  G  G  G  B  B  B  B  B  B  B  B

Each of the RGB components has 8 bits associated, giving 256 intensities of each hue. This allows 16,777,216 possible colours for each pixel. The reason this is called truecolour is that this is around the number of colours the human eye is able to detect.

Sometimes truecolour images are represented by a 32 bit value. The extra 8 bits are whats known as an alpha value. This represents the translucency of the pixel, so that when two images are overlaid, parts of the image can be seen through.

As CPU and graphics card power has increased over the last few years, 32 bit truecolour has become popular on the computer desktop to provide effects such as translucent windows, fading menus, and shadows.

See also: Planar, Chunky, Highcolour