Yellow is a condition of a rich air fuel ratio where the unburned fuel actually burnes in the headpipe. Allot of the time the chrome will actually bubble off of the headpipe. USUALLY the first 6~8 inches of the headpipe are discolored.
A Blue hue is a result of a lean air fuel ratio condition. Usually the first 8~10 inches of the headpipe are discolored and probably the collector area.
Even if the air fuel ratio is correct, some pipes WILL turn yelllow in the first 2~6 inches, this is due to the thickness of tubing used and pipe prep for chrome.
I saw allot of returns in my day. so I set up a test on the dyno one day to help out the warranty guy to prove what the colors really meant.
H-D RKC FLHRCI HDI EFI 2001: my "RIGHT" Screamin' Eagle Performance Muffler got yellow, but actually I have had no problem at all with it. My "LEFT" Screamin' Eagle Performance Muffler has NO discoloration, but the muffler itself literally DECAYED, with BIG/HUGE corrosive holes, just 3,5 years after the purchase (check out my thread "NEVER buy Screamin' Eagle Perfomance Mufflers! NEVER!") !