Here's one way to do this:
Define three custom value lists, each containing a single value (Red, Green and Blue respectively).
Place three instances of the field on the layout. Format each as a checkbox using one of the above value lists.
Reduce the fields' width so that only the checkbox shows (for added measure, set the text color to be the same as the background).
Excellent, just what I was looking for. Thanks! Now, I just need to see if I can play around with the way they are displayed, maybe make the checkboxes a bit darker, etc., and I think I'll have just what I was looking for.