That'd be a horrible, horrible idea.

Nicklist shows:

codemastr
codemastr
codemastr

the first one is just plain "codemastr"
the second one is "^k1codemastr" (ie, black text)
the third is "^k1,0codemastr" (ie, black text with a white background). I could come up with several hundred different combinations of this that would all appear exactly the same in the nicklist.

How do you distinguish the three? YOU CAN'T. Also saying "with some tweaking", well should mIRC support bitmap data in nicknames? Because I could "tweak" an IRCd to allow images for nicks. RFC1459 states what characters are allowed in a nickname. Granted, IRCds take some liberty on exactly what is allowed, but NONE of the "real" IRCds (ie, not ones made by script kiddies for use on there network where they want to look "l33t" and have colored nicknames) allow colors. Adding this feature would introduce bugs and problems, and it would add needless processing since 99.99999999999% of servers won't support it; it is a horrible, horrible idea.