Discoloration of chromed exhaust pipes (yellow, blue, purple, ...) are mostly from heat and not from an external factor.
Basically burned chrome-oxide on the surface.
The bluer it gets...the harder you are riding
Exhaust temp is higher right at the beginning of the header pipe. Thats why it effects this area most of the time.
For cleaning and rust prevention of everything chrome I use Nevr Dull.
It basicaly is a cotton wool soaked with a solution that cleans and also forms a thin protective layer after buffing.
Most riders seem to think they need Hi- Octane $$ fuel in their low compression Royal Enfields
The unburnt fuel over heats the poor exhaust valves and BLUES the pipe
Once I put race gas in my Briggs lawn mower The muffler turn CHERRY RED lesson learned.