I always go with black (or space grey) when I get a new phone because I feel like it looks the most professional. Having said that, I've always wanted to go with silver, gold, and now red. Can anyone relate?
Why would any color "look the most professional" over other colors?
Use the color you like most and, on professional situations, put a case over it.
A black case would look good on all of the colors....maybe a black Apple leather case or something along those lines.
My idea of the professional look would be Matte Black iPhone with a Apple leather case (black) .