It It depends on the breed of the chicken. Some hens are mix bred so they will tend to lay different types of eggs. It also depends on whether you have roosters to help the chickens lay eggs or not. If you do, then they might be helping with the different egg colors.

Ear colour is a good sign of what coloured eggs will be layed e.g. Brown ears = Brown eggs, Cream ears = cream eggs etc.
There are a couple of exceptions to that^.
Generally depends on the breed (What breed are they?)