I installed a Dyna muffler larger jets and a K&N drop-in. I started the engine and let it idle a bit. I actually watched the header turn blue. Not just a little, but a deep purple. Started at the head and sort for grew down the header.  Almost organic.

Anyway, someone here pointed out that I needed to re-torque the header bolts after installing a new gasket. I wnet and checked it and it was indeed loose. After retorquing it 3 or 4 times, it seems to finally be set. After that, the bluing stopped. I cleaned the header pipe and it's now a golden color.

So my point is that exhaust leaks will cause the bluing.  So re-check the torque of those bolts.

Steve