ive heard-blue is lean, gold is rich. I rejetted a 1987 zuki intruder, and it had yellow pipes befor i rejet. I would have thought they would be blue, thanks to the EPA keeping everything lean. 400 miles, and they are still gold.\ prolly just a myth. Hey! maybe mythbusters would be interested in this!!
It prolly has more to do with the temps of the exhaust, since yams, and the intruder have double wall tubes. too hot=blue/too cold=yellow.