AFAIK, the only way to do this (in an open standards kind of way, not like the way AOL tries to do it) would be to send the email as HTML, which IMHO is NOT a good idea. I also don't believe that IMP will allow it either.
My suggestion is if you feel the need to color part of a message, create a file (like a M$ Word document) and format it in that application. Then send that file as an attachment in an email.
Just my 2ยข