I have a Webra speed .61 muffler that is the same color...all I did was boil it to clean it out after I did a little filing in the port opening? I thought it had something to do with the heat, but maybe it's the Ph of the water? Or both???

Edit: It shouldn't be just heat related, because the exhaust gasses get a lot hotter than boiling water, aye?!
Maybe the chlorine in the hot water reacts with the alloy?