I think I might have a bad Bassani- bullet style cat,what to look for?

[Scotts 03 COBRA]

My check engine light has come on a # of times---my predator says O2 circuit slow response Bank 2 sensor 1( that is pass. front)I replaced that 02 sensor a couple of times,my check engine light still came on....I decided to take off the catted x and noticed the cat on the pass. side is darker then the driver side cat,which is the same color as the rest of the stainless x pipe---- Why is their a difference in color and the inside of the pass cat remains intact---Any info would be appreciated--------Thank you-----Scott