Re: Hard rubber discoloration
"I have a Selmer hard rubber mpc for my sop that is less than 3 years old, but has turned that lubberly milk chocolate color".
That, my friend is because it is a Selmer piece and it's not made of hard rubber, but rather.....fossilized SHIT!, and that is the reason for the brown color!
Ok, I'm joking! Come on, laugh! My guess is that this is a chemical reaction to the acid in your saliva. Try cleaning the mouthpiece with liquid dish soap and a tooth brush. You'll want to clean your mouthpiece with soap and water whenever you're done using it. I would also be sure to swab your horn a couple times and wipe it down with a cloth too.