You cannot use RGB for regular text. It can be used for things like picture windows, however. You can change the 16 text colors to other colors, but you will be limited to 16 colors. RGB or similar color options have been recommended on the feature request forum many times.
As a note, you could make a script that makes custom channel/query windows and does all the text work through the script instead of using the built-in windows and then you can use picture windows with RGB... but that is a LOT of work. I'm not sure if anyone has made something like that or not.
