Laura, if you want to study egg shell genetics, you might to check these out. In this thread, pay most attention to Tadkerson’s posts. He’s the expert.
https://www.backyardchickens.com/t/840867/clarifying-brown-egg-genetics/10
Then this article by Tim explains a lot. Have a couple of Tylenol handy though. You may need them. It gets pretty deep.
http://www.maranschickenclubusa.com/files/eggreview.pdf
The simplified version is that with a brown egg, the shell is white and the brown goes on last. Normally yes but not always. There is a gene that tints the white shell all the way through, not just puts brown on top. If you crack a brown egg, remove the membrane, and look at the inside of the shell, you will normally see white, but I’ve had a few that were tinted.
There are some genes, one dominant and one recessive, that restricts brown. I used to think that white or blue meant an absence of brown. Not always true. The brown gene may be there but if the gene is there that restricts brown, you could still get a blue or white egg. I told you that you’d need Tylenol.
The way I understand it, the blue egg shell gene is totally dominant, not partial or incomplete dominance. That means if just one of those genes is blue instead of white, the shell is going to be the same shade of blue as one with two blue genes.
Edited to add
I forgot to mention, both the brown and the blue come from recycled dead red blood cells. The blue color comes from a process similar to the process that makes the bile green/blue colored, but it does not come directly from bile. That's something else I had wrong.
