
The blue color is produced by biliverdin; the brown color is produced by protoporphyrin, and they are incorporated into the shell in different ways based on their different compositions. Again, to make it simple, the brown color can be thought of as being laid on the outside, while the blue color goes throughout the shell.

Green eggs contain both blue and brown pigments, and tend to be exceptionally hard to candle--harder sometimes than even some very dark brown eggs like those of Marans or Welsummers.