exhaust pipe discoloration

my one right side pipe is starting to turn a little yellowish brown, any reason for this? is this normal?

After market exhaust system or stock? The reason I'm asking is the stock system is double wall tubing and after market is single wall.

Year and model of bike? Did a sparkplug check? White is too lean,Black too rich and light tan about right.

stock exhaust havnt pulled the plugs yet just noticed it when i got back from a ride. its a 79 kz400 ltd