Yeah, wtf is that really?  
CGA 4-color mode is 2-bit graphics (4 pixels per byte), though people have come to use the bit width of the CPU to refer to game systems.  The SNES uses 8-bit color-indexed graphics, but everyone refers to it as 16-bit.  I forget what the NES gets.  It's like 32 colors at once, isn't it?  I don't know how it encodes its pixels.