you got some money there bro. APEXI then Greedy

Yes, it'll change color due to dust and smokes, wash it off everytime you clean your car.
The sound will Definitely Change overtime.
Just regular Clearning would help. Use some Metal Polish sprays, those things sure help, Dunno what's the brand though. It smell real strong, 100X bleach's smell.

I got HKS for a 1.5 yr now, I clean it everytime I wash my car. The inside, I wouldn't Clean much except for Dry - Cleaning with a towel (spray with that Metal Polish Spray), but only clean the portion of the Tip, wouldn't go in further. Now i'm Sick-n-Tired of Cleaning it. Just the Outside once a while.