I'm with bunji on this, its all about the patterns. I love fish with stripes particularly. But really any sort of interesting pattern, stripes, reticulated, spots, blotches, is what I look for. I also look for interesting finnage. I like fish that have unusual shaped fins, fins that are spiked or are long, that sort of thing. The finnage is why I got into killifish, they're just a little "different" than your average fish. If you looked into my community tank, you would't find much color but plenty of patterns and fun fins (Marbled hatchetfish, Threadfin Rainbows, one marbled molly, botia striata, panda cories, emerald cories (the only "color" in the tank), a spotted cory).