It could very well be a chemical reaction in the gas and have nothing to do with the tank. There are a number of additives and it comes in different colors from different gas stations. The spendier stuff looks even different yet. I even remember now that some stuff I've stored in my shed has turned darker and other stuff hasn't.
I don't know that it's significantly degraded though from a combustion standpoint just due to the color. You could always do some dyno tests and compare new gas to brown gas though and see what you find out.