I think dark green would look really nice, although I don't use a background on my tank. There's a light blue wall behind it, but the tannins make it look sort of greenish-brown.
My adventures in keeping Indostomus crocodilus, the crocodile toothpick fish. AKA armored stickleback, paradox fish, and freshwater pipefish. Care is (to the best of my knowledge) exactly the same as Indostomus paradoxus.