candles could produce discoloration from incomplete combustion.

You can wipe off some stains without discoloring the wall.

Mold that has been there awhile smells and discolors the surface. 

They use a premixed acrylic grout that self seals in 24 hours and if it discolors it usually just topical, easily cleaned.

It could discolor the epoxy and may not get it all out.

Upon moving in, I painted the walls and ceilings noticing discoloration.  

Discoloring and staining.We do a lot of high end remodels and builds.

The fire hazard is the biggest red light IMO, but also the smoke and butts will discolor everything, even if the tenant uses an ashtray, there's always the stray but will leave some kind of yellow/brown spots

For example, a roof can have discoloration and stains from a variety of causes.

And, as you said, if they poured it in with the drain nearly sealed, and the chemical sits on your porcelain very long it will permanently discolor it.2) We always do a physical move-in inspection, with written report that the tenant signs as well.