I have a set of laminated signal flags I tape onto the nucs. They are designed for people to recognize, but I hope there's enough for the bees to get. I also set them away from my regular hives. I tried requeening my full hives with dismal results, but luckily nucs were fairly good.One of the guys in my bee club suggested nautical flags. They come packs or strings that can be cut up. The can be stapled to the front of the hive. Nautical flags patterns are specifically designed to be easy to differentiate at long distance and in unfavorable conditions.
PS serves a duel purpose of numbering/lettering hives at the same time.