The colors used and the designs written all have meaning. There are geometric designs, ones from nature, floral, animal, etc. Colors play a symbolic role, too, and all these artistic choices vary per region.
There is evidence that these eggs were decorated way before Christian times, to ward off evil spirits and all, but about 988 AD with the acceptance of Christianity in the Ukraine as the state religion, the symbols took on new meaning. Basically the Pysanky eggs had their hayday in the 18th Century, with the oldest examples dating to that time.
Until now! Recent excavations in the Baturyn region of the Ukraine have found some egg fragments with decor that may well be as old as from the 1600's. The Baturyn egg is a completely natural, blown-out eggshell, colored with a grey-blue dye, and decorated with geometric ornaments. And even though the egg is crushed, it is archaeolically complete and can be restored. Baturyn was sacked in 1708 and all inhabitants slaughtered.
It will be interesting to see the complete restoration, and to know that someone before the terrible sacking and its aftermath, was happily engaged in an artistic pursuit, getting ready for Easter, as we Orthodox do to this day. Being Greek Orthodox, I don't make the Pysanky, but will be making the beautiful red eggs that we crack and enjoy on Easter morning.