You seem miffed by my remark.
All the elektronauts I know who often use their elektron machines have had some hue of yellow, brown or green on one or more of their encoders at some point. Some leave them the way they are, some replace them. To me it's a sign of experience, usage and that you might have actually played a sweaty gig or two. Rock'n'roll baby!
If it bothers you,
A - follow Simon's advice and buy a new set.
B - I put some black ribbed ones from another device on my OT.
Or, C - as a friend of mine suggested, you could pop the dirty ones in the dish washer on a full moon, after midnight, and recite the OT manual backwards six times while standing on your head. He said his life changed for the better after he did that.