For unmarked ropes a handy short term trick is chalk. Sounds dodgy but it works much better than I thought it would when first heard it suggested.
Sure it wears off relatively quickly, but do it on the first of 10 raps and there's a good chance it'll see you down, and you can always freshen it up.
Of course if you manage to get chalk anywhere else on your rope... try doing something distinctive, two or three bands say. |