I don't know about the Welsummers, but you can scratch the color off of a Marans egg. I think you'd have to do some real scrubbing to wash it off...it doesn't come off that easily...but if the eggs get rolled around in the nexting box, say, you will see the scratches on the egg. It really is like a coating of paint!
I've gotten a few weird eggs that missed the "paint" entirely (or just have a spattering of speckles over it) and those have looked just like your ordinary brown egg as far as darkness goes. But on a day-to-day basis, even the lightest of my Cuckoo eggs is much darker than your typical egg. I'd say the lightest "normal" Marans egg I've gotten has been a 4, with 5 being average--and the occasional 6. But this is my hens' second year of laying, so it is to be expected that they will be lighter. Their first year I got mostly 6s and 7s. There's a pic on my Marans page on my web site. Wish they laid like that all the time! But that's part of what makes the breed challenging. Another bit of Marans trivia: if a hen doesn't consitently lay a "4" on the Marans chart, it's not a Marans (and should not be used for breeding) no matter what her parentage is.