My africans will do this a lot like someone stated, I think it's a way they can size each other up more aggressivly then just doing circles with their fins up. Then when they really want to they start bitting and chasing then if they want they kill the fish. I have one chromis and plan on buying 4 more.
In the store his top half had a light tint of the green and blue and the rest was a silver, he didnt look that nice to be honest. I've had him for a few weeks and he looks amazing now. He never had color on his tail fin and now that and his body is just glowing with bright green and then blue at the right angle, very nice fish, also he is very active and friendly, always coming to the top and swims back and forth when he sees me come up to the tank for food. Zooplankton soaked in vita chem and osi flakes is what I use as their main diet.