No he's not. He is an old friend of Ghetsis who was brought into Team Plasma (The black clothed ones) to assist Ghetsis in making the Plasma Frigate, studying how to strengthen Pokemon (His DNA Pins for Kyurem), and to study Kyurem.
When you defeat Plasma, Colress stays incharge of the small group that was with him and docks somewhere for you to battle him. He just never agree'd with Plasma's ideals to begin with. He was only interested in assisting them because it played a part in his actual research on the Strengths of Pokemon.
He doesn't have any affiliation with the White Knight Plasma members.