Well, I tried black, and I've tried grey, and those schemes blend well into my grasslands and ruins terrain, but my initial reason for buying Tyranids in the first place, was for space hulk style battles in corridors and rooms of a space colony. When I put black or grey Tyranids on my corridor piece, they camouflage so well with the floor and the dark/medium toned walls, that they are almost invisible - you can't make out any details of the figure when it's in the corridor.
So, I tried yet another paint scheme today, trying to think of something that would be more visible inside the green and metal corridors and rooms, but still be a dark type of paint scheme. I started by painting the whole thing mithril metal, and then applied multiple coats of purple and blue washes to the figure, leaving the claws raw metal so they stood out. The metallic undercoat beneath the colours gives it a kind of shiny reflectiveness that I feel is reminiscent of bugs, like a cockroach :) Anyway, this is the result of this paint test.