NTSC Palette
There are in effect 128 different colours, as D0 of colour registers is unused. The image below shows the results of the 256 possible values that can be selected. Although each pair appears to be slightly different, this is an optical illusion. They appear in pairs because D0 is ignored and so (for example) colour $34 and colour $35 are identical. Colour number goes down the rows, and luminance across the columns (so, row 3, column 4 - both counting from 0 - is colour value $34 and appears red on NTSC systems).