Black 0 0 0
Red 255 0 0
Green 0 255 0
Blue 0 0 255
Cyan 0 255 255
Magenta 255 0 255
Yellow 255 255 0
On low end monitors, eight bits must do for all, using a color map to translate from bits to colours. A common core of 216 = 6 x 6 x 6 colours is common to many applications, allowing 6 possible values to each of the three basic colours. These are the multiplies 0, 51, ... , 255. Each may therefore be indexed by an integer in the range 0 .. 5. The indices represent fifths, so 1 2 3 represents RGB values of 51 102 153 out of 255 255 255.
The possibilities coordinate a cube made up of 216 smaller cubes. I have displayed below sections of this cube, sliced by planes ranging in turn from dark to light. These are the 216 "safe" colours which ought to be displayed without dithering in your browser. Nonetheless I know from experiment that they will be dithered in certain circumstances.
