Blue eggs come from a handful of breeds
A blue shell is blue all the way through, which you can check by cracking one: the inside of the shell is blue too, unlike a brown egg, which is white inside. That trait traces to a small set of breeds. Araucanas, the rumpless, tufted originators, and Ameraucanas, bred from similar stock with tails and muffs, lay reliably blue eggs. The Cream Legbar, a British breed, lays blue to greenish-blue eggs and has the bonus of sexable chicks. Then there are Easter Eggers, which are not a breed but mixed hens carrying the blue-egg gene; they are friendly, varied, and unpredictable, laying blue, green, or tinted eggs, and they are what most hatchery 'coloured layer' offers actually are.
Dark brown and speckled shells
Brown eggs get their colour from pigment painted onto the shell late in laying, and the Marans, a French breed, lays the deepest shade: dark chocolate eggs that make ordinary brown eggs look washed out, with the darkest shells usually coming at the start of a hen's laying cycle. Welsummers lay a rich terracotta egg, frequently speckled, which looks beautiful in a mixed basket. Barnevelders and Penedesencas also lay notably dark eggs. Be aware that dark-shell breeds generally lay fewer eggs than production breeds, and individual hens vary: one Marans may lay near-black eggs while her sister's are merely dark brown, because pigment intensity is partly individual. Shell colour also breathes across a laying cycle. A Marans usually lays her darkest eggs in the first weeks after she starts, or after a moult, and the shells lighten gradually as the season runs on, because the pigment supply is spread over more eggs. None of this signals a problem; it is simply how the paint pot empties. If dark eggs are your priority, judge a hen by her early-season shells rather than by what she lays in late summer.
Green eggs and mixed parentage
Green eggs appear when a blue-egg layer is crossed with a brown-egg breed: the blue shell shows through a coating of brown pigment, producing shades from sage to deep olive. Hens bred deliberately this way are sold as Olive Eggers, and crossing an Olive Egger back to a dark-brown breed deepens the colour further in the next generation. This is where coloured-egg keeping becomes genuinely interesting for small breeders, because egg colour is inherited predictably enough to plan for: blue is dominant, brown is built from many genes, and each hen you hatch is a small experiment whose result appears with her first egg, months after she hatches.
Colour is decoration, not nutrition
It is worth saying plainly: a blue, green, or chocolate egg tastes like a white or brown one from a hen fed the same way. Yolk colour and flavour come from diet, not shell pigment, and no coloured egg is more nutritious than another. Choose these breeds because a multicoloured egg basket is a daily pleasure and because they make eggs easy to give away and sell at the gate, not because of health claims. Because parentage decides everything about the next generation's shells, paperwork has real value here. A hen whose KinsLe profile travels with Transferrable Profiles brings her breed, parentage, and the shell colour she actually lays recorded by her previous keeper, which is exactly the information a coloured-egg flock is built on.