When I am overhauling a horn with white roos, I wash my hands every 20 minutes with that orange hand cleaner stuff that really scrubs you clean. If I get a pad dirty, I throw it out. Its a bit of a pain, but they sure look nice when they are installed.
As far as gunk on the pad AFTER an overhaul, I think you have three options, presented here in descending order of feasability for someone who is not into repair themselves:
1. Gently rub as much as you can off with an untreated plain paper towel or cotton q-tip and leave the rest. It will look nasty but if it is just a smudge and not actual material, it probably isn't affecting the seal. Definitely check with a leak light to make sure.
2. Take a cotton swab or pipe cleaner with some naptha on it and gently wipe away the smudge. It will be clean, but you will probably decrease the life of the pad relative to the others. I have done this before, and as long as you aren't scrubbing the pad and just wiping it, it works fine.
3. Replace the pad.
I have seen white roos on a horn that was done a while ago and some of them had turned green and brown from various alcohol-tobacco-saliva mixes. They still sealed fine, but it wasn't pretty.
I also had white roos on my Aristocrat alto for a few years before I traded it away, and they looked as good as the day they were put in, and I played it quite a bit.