I am an IT Professional working for Microsoft and we have all kinds of color Lumia's around the office. None are less professional than others. I, personally, have a Yellow one and would not trade it for any other color. Does not make me any less professional than someone with a black one, or any other color.
One thing I do have to say though, is people DO take notice of my yellow one, even other 920 owners and several have stated that now that they have seen a yellow one in person, they wished they would have gotten one as well.