Well, I’ve not tried this on a set of porcs but I did have some success on some vintage “NOS” white road tyres that had storage marks so I don’t see why this shouldn’t work.I doubt there are that many on here that have had the chance/need to clean their white porcs in modern times given their rarity and apparent desirability, so guidance may be a bit lacking!
However I would say that good advice may come from the likes of @Peachy! or @regan_ev who have history in rejuvenating discoloured retrobike trinkets.
There’s loads of videos and posts in the vintage trainer world on how to restore/rejuvenate white rubber soles
As Betsy has suggested isopropyl.....I used to do injection rubber moulding.....white was feared.....any staining ruined mouldings so we'd use isopropyl to clean the fresh milled uncured rubber before moulding. It evaporates so quickly that's how it's lifts dirt. Use clean plain cotton...as lint free as poss.
Then when they're clean get rid of them before they get dirty again![]()
What a great read for a retro community. Top work to those chemists and all those who were willing to partake to help perfect things.