If you look at the inside of the egg shell, it's either white or blue. Gifa is right. Even brown eggs are white inside unless they carry the blue egg gene, then even if they are olive on the outside, they are blue inside.From what I understand there are only two colors of shells, white, and blue. And this is only because the blue pigment is introduced to the new white shell at the beginning of the shell formation and thus the pigment penetrates it more thoroughly. Whereas brown pigment is added late, and only coats the shell, whether that's a white shell or a shell that has been pigmented to be blue. I have never heard of a plum egg other than as an additional descriptor to the shade of brown. The more brown pigment, the darker the brown... if it is over a white shell, it's a brown egg, if it is over a blue shell it is a green egg, and again, the more brown pigment, the darker, more Olive the green.
So... I am not sure anything has been exacerbated.
If the hen laying the egg caries two blue egg genes, her eggs will always be blue inside. If she carries 1 blue egg gene and one brown one, her eggs will be blue inside, if she carries 2 brown egg genes, her eggs will be white inside.
At least that's my understanding of the genetics behind it, and I have not found an exception with my eggs.
Willow