Most of us don’t like to think of our hedgehog visitors being artificially marked, but sadly that is most often the explanation for white markings on the spines. People use all sorts of different subtsances, it seems. As it sounds as if you have found, it is possible to recognise individual hedgehogs by their natural markings so that it’s unnecessary as well as inappropriate to artificially mark them.
Lovely to hear about Harriet, Piglet and Al. I hope they are all doing well.