You will need to learn some RTF specifications. Then iterate the input text and set opening and closing rtf tags to set the color. Also, keep in mind that if you have added the rtf tags you will need to verify if you have already added a tag so you aren't adding the same tags around specific characters multiple times.
If there is another way I am not aware of it. I haven't used Richtext box in a couple years so maybe there is another way???