See the how-to on the wreath project here; it's essentially the same process for the eggs, except of course you start with some plastic eggs (mine was a 99 cent set from Target) rather than a wreath form. I took my color inspiration from the eggs, that way if a little of the plastic peeks through, it doesn't totally stand out. Again, I used two close shades of the same color to give it a little depth.
I decided to cover all the way, rather than leaving a blank spot at the bottom for them for balance, but they seemed to stand up okay anyway. I saw some giant eggs at Michael's that would be fun to do for a bit centerpiece. You could also attach a ribbon hanger to the top before hydrangea-ifying.