Even within a breed some hens lay darker than others. My barreds lay a light to medium brown egg. My partridge rocks slightly lighter, except for a couple who lay darker. My blue rock a medium brown egg. My Delawares lay a brown pink egg, some lighter some quite dark. Answer it varies HIGHLY dependent on your stock lines. Some dels will lay a speckled egg.
If you want a truer dark, cuckoo marans often through a nice even brown egg. Then many others do lay the near chocolate egg they're famous for, and everywhere in between.
Get photos of the eggs someone is getting from any specific hens they have. A parent's egg, even the egg the rooster came out of help predict the next generation's color ability. A roo from a nearly white egg isn't going to help produce good egg color in his offspring. I only hatch the darker eggs here and it's helped a lot in two generations.
I like the rainbow egg thing too and am getting set up to add a few colored eggers here as well. Was considering pure white layers too just for the comparison thingy.
What colors are you shooting for? White, blue, pink, brown, green, speckled, terra cotta, chocolate?