There is such an array of colorful fish that will go with your original collection. Other Gouramis, Most Livebearers ( Mollies, Swordtails, Platys), Some of the med sized Tetras, Redtail or rainbow sharks, Loaches and some of the smaller Cichlids like Rams or Apistgrammas are just a few. All these will have either brilliant colors or fascinating patterns to them. ( I've always been a sucker for YoYo loaches
) You just want to get fish that are size appropriate to your current fish. Too small and they might have a problem
Remember also that your tank should have fish that use all 3 layers of the tank (Top, Middle and Bottom). By using this method, you won't run into the "Not enough elbow room" situation that happens when all the fish want to be in the same layer. (No matter how much you try, a bottom layer fish will not become a top layer fish
)
For color you could also add driftwwod (for appearance) and colored artificial plants as well as some decorative colored rocks. There are just so many ways to get color into a tank these days.
Hope this helps