All eggs originate white and get different color tints applied throughout the cycle. Brown eggs get their color applied late in the cycle and it's applied only to the outside of the egg, so the inside is still white. The blue shades are applied early in the cycle so both the inside and outside...
They do change throughout the season. I have both Ameraucana's and black copper Marans. The Ameraucana's start the season laying greenish blue but as the season goes on they turn more lighter blue. The Marans start dark brown and lighten up later in the season.
Prairie Bluebell Egger lays baby blue eggs. Much lighter than any other breed. I thought my Ameraucana's eggs were a nice blue till I got the Prairie Blue Bell Eggers.