If you look at your axolotls, and see a smattering or slightly dark patch (sorry can't think of a better descriptive word!) on your axolotls head region, then chances are they'll develop them in several months time. Otherwise they may only have a light smattering/bit round their eyes. Two of our leucs only have rings round their eyes. The other 3 leucistics burrow their heads in sand snuffling, so the majority of their dark colouring comes from sand (only realised after they'd been in the fridge and all the sandy bits came of as we did regular waterchanges, apart from the tiny dots round their eyes and tiny smattering of grey on their head.)