In a marked area on the upper wing is thin hoarfrost allowed, ice build up is very common there especially if the fuel is cold from the previous flight. We had serious problems in the beginning because the planes iced up in the canaries and it took up to 8 hours until the frost was completely gone, no deicing equipment there of course. Boeing OKed it and our local EU-authority is happy with that.
On the lower side of the wing icing up to 3mm is allowed as it was already on the classics.