hmm, i don't quite get what you mean... because an 8-bit color = 2^8, which already produces 256 colors..
a 16-bit color = 2^16 = 65536 colors...
a 32-bit color = 2^32 = 4294967296 colors!
i think photoshop and gimp can "auto" select their color tables to limit the colors to 256 or something..
example from this video:
RE: Colors Distance - Measurement & Visualization