Here is how the standard describes the egg:
Welsummer eggs should be of good size Colour: A rich, deep red-brown, as dark as possible. Some products are speckled and occasionally blotched. Glossy eggs are produced but the matt egg is the preferred.
I find that my darkest eggs are about a 4-5 on the Marans scale. But one significant difference from a Marans is that lovely terra-cotta (red-brown) undertone that you don't want to lose by aiming for a dark chocolate brown. Here is the batch I just set from my hens who have been laying since December.