I used to keep my male dark blue betta in my community tank, with 3 platies. 1 of the platies is a bright orange mickey mouse, and the other 2 were black platies. One night my betta bit every bit of fin and tail off the black platies (they didn't make it), but he left the orange mickey mouse platy alone. Apparently this was because the black platies reflected a lot of blue similar to my betta's colour, and so he saw them as competition. I've given my betta his own tank now, but he stayed in the community tank for a few weeks after killing my platies - he didn't bother any of the other fish whose colours are nothing like his own. A guy at my LFS told me this is common, and that as long as platies and bettas aren't similar in colour, they usually get along just fine.