It has been at least 10-11 years since I attended my last show, so it sounds like Marans have come up in the world. They weren’t as popular in show at the shows I went to and there were only like 4 people there showing birds.
I believe type and size is very important also, in layer and dual purpose breeds. Because in layers you want them to have a wide and deep enough pelvis to be a good representation of a layer breed. And in dual purpose breeds you want a bird of adequate size because the breast and thighs should be big enough on the carcass to make it worth while to eat them.
In breeds with specific traits, like certain egg colors, such as Marans, I feel like it is dishonest to knowingly show a bird that is not a good representation of the breed’s well known dark brown egg color. But as you mentioned, you can’t judge a rooster by egg color so only the breeder would know. I don’t plan on showing in the future, but I can see how it would be enjoyable if you had a more well-known breed that was judged accurately by their SOP.
I don’t breed Marans anymore, not because they didn’t show well but because they truly are mite magnets with all their plumage. I have never owned a breed before that I had so much trouble keeping mites off of them. I spent so much time dusting them, spraying them, and treating their perimeters that it took away from the pleasure of keeping them. I prefer tighter feathered breeds like Legbars, Leghorns, and Games which all seem to be very mite and bug resistant. It is pretty rare we find mites or bugs on any of those breeds just because their feathers are easier to keep dusted naturally by the birds.