I'am almost sure that technique came from Russia or another East Europe country because my mom (from Ukrane) every Easter paints eggs in that manner And all russian housewifes apply onion skin for that purpose!
Here are some another natural dyes for eggs:
dark brown - boil eggs in water with black tee or birch leaves.
red - add beet juice
bilberries and cowberries give lilac and pink color
dried (or fresh) spinach, parsley give green color