I was wrong in my previous post. one byte is 8 bits not 4. I've corrected my previous post.
it would take up a considerable amount of room in the block data, I'm not sure if there's any room left.
Context:
3 bit color (1/1/1) (38% of a byte, or 13% of block data) gives you 8 colors
(Primary and Secondary colors. red, green, blue, yellow, purple, orange, black, white)
4 bit color (50% of a byte, or 17% of block data) gives you 16 colors
5 bit color (63% of a byte, or 21% of block data) gives you 32 colors
6 bit color (3/3/3, or in binary 11/11/11) (75% of a byte, or 25% of block data) gives you 64 colors
6 bit color would give you the most customizable colors, as you would be able to select how much red, green, or blue you want in each color. It would allow you to select 0%,33%,66%, and 100% for each color.