I think "at Easter" sounds the most natural, but any of these prepositions work. "For Easter" perhaps suggests we are painting them beforehand, "during Easter" suggests it is a prolonged activity or that it occurs at an uncertain time at some point over the Easter weekend, and "on Easter" is less natural but fine enough.
"at Easter(time)" is also acceptable in American English. It is better than "on Easter" because "on" is on that one day, but "at Easter" or "at Eastertime" gives a little leeway. I'm sure some people will kneejerk and say I'm wrong, but just think of "at Christmas(time)."
"i påsken" means during eastern. There is no such thing as på påske, but "til påske" means for or to eastern; before the holiday or to be used during the holiday.