There is no restriction as to what colors to wear. My each turban matches with my shirt, and I change it everyday. I have seen people wearing some turban everyday, i mean the same color, why. In a professional work environment you need to change clothes everyday, why not turban after all it is a part of your dress. We have to remember one thing, each singh represents all of your community, and the image he or she creates is for all of the sikhs not just him. I have been recognized by so many whites and given good comments to dress professionally specially the turban always matches with the shirt or pant. I still remember one of the managers from Deloitte when I use to work with him, he had someone in his last project, a sikh , whose turban use to stink, and he use to wear the same one everyday. I felt so bad. He use to be surprised to see me with different colors matching with my shirt everyday. He was under the impression that sikhs wear the same color everyday and they are dirty.
Guys use your common sense. Be professional, there is no restrictions as to what color a sikh should wear. Look good, as it is a materialistic world, everyone is concerned about looks, so we have to show that even sikhs can look good.
Thats my opinion. Rest is your judgement.