you can change your colours locally with Alt-K (colors dialog). Alternatively you can use an ON INPUT script to change your text as you type it. Note that most IRC users *HATE THIS* and will likely get you in lots of trouble, so I wouldn't suggest doing it. If you don't want to listen to me, you can use this:
ON *:INPUT:#:if (/* !iswm $1- && !$ctrlenter) { .msg # $chr(3) $+ MYCOLORHERE $+ $1- | halt }
Change MYCOLORHERE to your colour but if it's only 1 digit prefix it with a 0 (use 04 not 4) and place it in Alt-R (remotes).