I find them impossible to distinguish. I should add that I "cherry picked" the best eggs from each pen to make the comparison, so this represents the best from a weeks collection. The sample sizes varied widely, lots of Welsummer and Welbar eggs to choose from, but very few Marans and BF's.
The Marans are clean because that is a small flock to they are in my smallest pen and that happens to be in an enclosed barn, very protected from the rain and snow. It is dry to the point of being dusty in their pen. All the others are in my large coop with no floor (straw on bare soil), so they tend to see a bit more mud when it rains.