37 Gal - what to stock?

well, it depends on the dimensions. You could put some dwarf mbuna such as Cynotilapia afra or Psuedotropheus saulosi/polit/demasoni in there if it is longer/wider than it is tall
if not well maybe bolivian rams if they aren't considered Micro

good luck!

What type of tank is it? Planted or hardscape?

Pair of convicts/HRP or some shell dwellers with topwater schooling fish would be my choice. Keep in mind how tall this tank is, and how a 40breeder or 30breeder would better suit cichlids due to it's larger footprint.

How about a pair of
They start out polka dotted white and black, and turn into black purple and white.

gonna be hard to have "big and full-sized" cichlids in a 37 gallon. I would say apistos, german blue rams, bolivian rams, kribs, pair of convicts, firemouths (which may actually fit your bill....fun and pretty and decent size). That is about all I can think of right now that would go in that size tank.

The firemouth's came to my mind also. If you were wanting something a little more peaceful that could be a centerpiece for a community tank, what about a couple keyholes(Cleithracara maronii)? They remind me of puppies lol, slow growing puppies at that too. Probably want at least some fake plants, places to hide and a school of larger tetras or such. They are a rather shy and peaceful cichlid that stays smallish for S American(5" or so).