16.7m colors is an 8 bit (256 colors for red, 256 colors for blue, 256 colors for green) display.
256x256x256 = 16,777,216 colors. A 6-bit display uses dithering and can only manage 16.2m colors (with dithering). 6-bit means 64 colors per primary color. 64x64x64 = 262,144. With dithering...