How to get egg shell colour right?

The colour of the egg shell is genetically determined and therefore strain specific. Yet, within a flock you still see considerable variations in shell colour. The colour in itself is not very interesting, the unexplained variation in the flock is more important. Sometimes you can see a relationship between production levels and shell colour. The more eggs a hen lays, the lighter the shell colour. But there are also certain strains that are very good layers and still maintain a dark brown colou