When I made the choices of tubing at the time Tygon was the best choice for tubing.
But could you explain why only the tubing connecting to the res have the colored problem?
I know it's dye and possibly the tubing added to the problem (mind you if you scrape the crap off and add water it's basically dried dye).
Could the light source, tubing, and dye cause some weird buildup located around the res?
(Silver coil is in place)
The tubing went bad from CPU block > Res > Pump...