They blacklight a cool white, showing they have been exposed to very little UV being stored in boxes. They will intensify with exposure to UV. This particular set was at the bottom of the bin, laying on top of knob set screws. Some of the screws oxidized and stained these knobs. I never tried to clean them.
It is possible CLR will remove the stains, that is what I used to clean rust stains on my counters. They all match and will continue to age together as a set. These knobs are good up to 1975, but the nylon material changed in the early 70's to a more translucent grade (looks like skim milk). These are the 60's opacity and color.