The base color of an egg is either white or blue. That's controlled by genetics at one specific gene pair. Blue is dominant so if just one of the two genes at that gene pair is Blue the base color is blue. If the hen has a blue egg gene she pulls pigment from the bile to color that eggshell blue for the full thickness. If she does not have a blue eggshell gene the entire eggshell thickness is white. It is not that she is coloring it white, she is not adding any pigment. The eggshell is made of calcium and calcium is white.
Brown or Green is just brown pigment added to the outside of the eggshell in the last half-hour or so the egg is in the shell gland. That's why the inside of the shell is either blue or white, the brown pigment doesn't get down that deep. You can scrape that brown or green off with sandpaper or even rubbing it with your thumb if you are firm.
@Chubbicthe2nd You are looking at them. Are those grey eggs the same color all the way through the shell or just on the outside? My guess is that they are the same and something has happened to that hen to stop her from extracting the blue color from the bile. I've never heard of that happening, I don't know why it would.
Yes, they can change shade over time as far as that brown pigment goes. That is normal. That brown goes on at the end of laying process in the shell gland. If something happens so the egg is laid a little early all the brown may not go on so what is normally a dark brown egg looks pretty light or even white. What would be a normally dark green egg can come out bright green or even blue.
It is pretty common with my chickens that the first eggs laid after a molt are pretty dark. (Either dark brown or dark green.) But as the year goes on those eggs gradually become lighter. This is established hens, not just new layers. The amount of brown going on the outside of the eggshell lessens.
Some changes of shade of the color are pretty normal. But they don't go from base blue to base white.
