as metal heats up it changes colors,,on headers cars and bikes,,they get hot enough to make them glow red,,when that happens and they cool down they will turn blue, one reason it does this is if the engine is running a little lean then the exhaust temp is higher,,,the headers on the bike is a double wall pipe, thats why they dont blue,,
I was told by Mark with Horseapple Ranch that Honda's OEM's are double walled and a lot of after market pipes are single walled.