It seems like anytime computers or video games are mentioned, somebody ends up talking about bits. Both processors and colour palettes can be measured this way, but what it means is less often received rather than an obscure idea of where in time something might have come from in time when someone says, for example, 8-bit.
The general rule of colour bits is the number of variations in a colour palette according to how many bits it is. In 4-bit colour, you have four base colours (greyscale, red, green, blue) and the variations of those colours are sixteen, as is four times four, and the palette becomes the series of bars below.
8-bit is a step up from 4-bit in that it is four times the amount of colours. Where 4-bit ends up delivering sixty-four, 8-bit has sixty-four variations of each colour.
Up, and Out
Furthermore, 18-bit colour follows the same rule, presenting three-hundred-twenty-four variations of each of the four base colours. Additionally, at least in the same resolution, the bars become much less noticeable.
Typically regarded as 'true colour,' 24-bit is a step up from 18-bit, with five-hundred-seventy-six colour variations of the base greyscale, red, blue, and green. The colour bars nearly disappear and a much smoother blending is therefore allowed.