The original Amiga OCS chipset had a palette of 32 color registers which was the maximum that could normally be displayed at the same time, using 5 bits per pixel to index them plus 1 bit that could be used to halve the brightness of the color.
In Hold-And-Modify mode these 6 bits weren't used to encode a color, but a command. You could set the value of a pixel from a 16 colors palette, or copy the color of the previous pixel while changing one of its components. 2/6