For yellow, get a rock of yellow polyps. They are easy to keep and spread fast. You could also get a yellow gorgonian or sponge.
For red, I can't resist red shrooms! My colt is dark pink/light red. If you have the lighting for it, get a RBTA. They are nice additions, espeically if you have clowns.
I know it is a short list, but those polyps, shrooms, and colt grow really fast and can take up a large amount of space in a short amount of time. :happy: Good luck and keep the pictures coming!