What’s happening to the metal is that it is getting so HOT it is being a heat treating processes. And different metals react differently to certain temperatures of heat, and apparently the metal they use for our aftermarket exhaust systems turn this color at the temperature of what the header is experiencing. So I wouldn’t be too worried about the color, just check the spark plug and make sure there inset any white or grey sh*t on it, just to be sure that the motor isn’t running lean. But if you want to feel safe, run it a little richer.