For color, your best bets are the following smasher species (followed by maximum size and my subjective colorfulness index from very to dull). All of these animals are at least occasionally available from commercial sources and can survive in a well-run in 10 gal with proper filtration or lots of established live rock.
Gonodactylus smithii , 8 cm, very
Gonodactylus chiragra, 9 cm, moderate
Gonodactylus platysoma, 8 cm, colorful
Neogonodactylus wennerae, 7cm, dull
Neogonodactylus curacaoensis, 7 cm, very
Neogonodactylus oerstedii, 7 cm, moderate
Gonodactylellus viridis, 6 cm, dull
Gonodactylaceus ternatensis, 10 cm, very
Gonodactylaceus mutatus, 7 cm, moderate
Haptosquilla stoliura, 6 cm, moderate
Haptosquilla glyptocercus, 4 cm, moderate
Odontodactylus havanensis, 7 cm, colorful
Odontodactylus brevirostris, 7 cm, colorful
Odontodactlylus latirostris, 7 cm, colorful
Echinosquilla guerini, 8 cm, colorful
Obviously some of these species are color polymorphic and some morphs are more colorful than others. Also, I gave maximum size, but I would not put an animal greater than 7 or 8 cm in a 10 gal system. On the other hand, all of these species grow very slowly once they reach 5 or 6 cm.
Roy