Well, colorful is in the eye of the beholder. O.S. are pretty much the Coke (or Pepsi) of the mantis world in the hobby color wise. There are some real sweet looking Squilla mantis, but they are spearers, and some of them will lose their colors in your tank.
I like the G. Smithii and the G. Chiragra in addition to the O.S. Pretty sure they both stay a few cm smaller adult size. Not sure exactly where you'd get either of these guys, some luck out and get G. Smithii on their liverock. If you want a small smasher, you could always go with the G. Wennerae. Easy to acquire, they come in Florida liverock from TBS and Gulfview. Both places will sell you one without the rock. They stay small, under 3" and are quite attractive. One of mine is lime neon green and the other is brown/green with reddish trim.
HTH.
S !