Well, after visiting all but two of my neighbours, because they had nothing to tap on, I feel reasonably confident in that I can explain common and uncommon eggs.
Common eggs are the colour eggs you find most often in your Springfield or whilst out visiting neighbours. So that means if you find more blue, than you do pink, your common egg is blue. If you find more pink, your common egg is pink. Seems simple enough.
Uncommon eggs are basically the opposite to common eggs. They are the eggs you find least often. Meaning, if you find blue a lot of the time, your uncommon egg is pink. It the opposite if you find pink more often then not. Your uncommon egg will be blue.