Colour support at the terminal client is not very easy to work out. Using the default windows colours works reliably. All other colours you need to test individually.

We wrote about this in FoxTalk, May 2000, I think.

Since then there is a new version of the client software, but only for XP (and the 2003 Server beta) which supports audio as well as more colours.
The more colours you use, the more bits per pixel and the slower updates will be.