You can use a ton of onion peels and boil them for like an hour to make a red dye.
But that is not what we did The other thing you can do is wrap an onion peel around the egg (but not everywhere) and then wrap a piece of nylon around it and tie it really tight. Then you boil your eggs in the dye for about 15 minutes (you can use the natural dye mentionned above from onion peels, or from beets, or special egg dye, or food coloring, although with that last option you need likt three bottles and it stains your fingers. we used egg powder coloring). Let them cool down and a couple of hours later you remove the eggs, the nylons, the peel, and you have a pretty marbled pattern on your egg in orange and yellow tones where the onion peel was (see the middle picture I posted). It is gorgeous on blue eggs, but red eggs are traditional and I am too lazy to make multiple colors.
When you wrap, instead of onion peels, you can use herbs and they leave their print on the egg. We used cilantro, italian parsley (last picture) and bay leaves.
You just buy cheap nude nylons and cut them in pieces.
ETA: I googled it and this has pretty good pictures explaining how it is done