I have yellow as well but none of them turn black on the face when "excited". However i have read about "bearding" in this species and from what I remember it doesnt mean any type of hybridization has occured as it is naturally occuring with some. I may be wrong but might be worth Googling it and see what you can come up with.
He is in with 2 other yellow labs a venestus and a sunshine peacock. I really dont think this is from stress he rules the tank. He isnt showing any signs of stress and i have checked my water and evrything is within limits.