It's the difference between 16 bits per pixel and 32 bits per pixel in VRAM. A screen using 18 bits per pixel (6 bits per color) would need the 'millions of co

It's the difference between 16 bits per pixel and 32 bits per pixel in VRAM. A screen using 18 bits per pixel (6 bits per color) would need the 'millions of colors' mode unless Apple designed some clever hack.