RGB Colour

Every colour a screen shows is made by mixing red, green and blue light. In an RGB image, a pixel's colour is stored as three separate values — how much red, how much green and how much blue — and each value is held in its own group of bits.

Splitting the bits into three groups

The bits stored for a pixel are chopped into three groups in a fixed order: red first, then green, then blue. Each group is just a binary number, so it records how strong that component should be. A group of all 0s means none of that colour, and a group of all 1s means as much of it as possible.

Adding up the bits in the three groups gives the colour depth for that pixel.

That pixel is stored as 101110011, so its colour depth is 9 bits: 3 for red, 3 for green and 3 for blue. Red and green are both high while blue is lower, so the pixel looks yellow-green.

Why three groups?

Your eye detects colour with three kinds of sensor, so any shade can be built from different amounts of red, green and blue light. With 3 bits per group each component has eight possible strengths, so the steps between shades are fairly wide. Giving a group more bits creates finer steps in that component, so the mix can be matched more closely.

When every bit in all three groups is 0 the pixel is black; when every bit is 1 all three components are at full strength, which gives white. The groups do not have to be the same size — a system could use 5 bits for red, 4 for green and 3 for blue.