It is done by wrapping the eggs in onion skins before boiling. Thos are the brown, amber and yellow ones. The pink/purple/red ones are already peeled in the photos, and they are done by cracking the shells of previously hard boiled eggs all over, sort of crunching them gently without peeling and then soaking in tea (or in my case, beet juice) for about an hour and then peeling.
I usually place ferns and flowers and leaves on my eggs, wrap them tightly in cheesecloth or old other thin fabric scraps (nylons work really well) and then boil them in coffee, tea, beet juice, or water with red cabbage and they come out beautifully. I was thinking of making an instructable in time for easter this year and while looking to be sure it was not already here I found the fabulous onion peel instructable!
Here's my results. Not too shabby for a first try done in a hurry, if I do say so myself. Thanks to the authors for the great instructables!