On Bev Davis' web site she states the rule this way I believe; the bird has to lay at least a #4 on the chart at some point in the lay cycle or it isn't a Marans.
Not a very high mark to achieve when you think about it. However, remember there are many things that can effect egg color. And, of course there is the standard for feather color and confirmation that have to be paid attention to as well. But I think the point is, that if the bird NEVER lays a egg of at least #4 or higher, then it doesn't meet the standard to be called a Marans.
The egg color is a primary genetic characteristic of the breed and it is the first item written in to the standard of perfection:
"Characteristics of hatching eggs:
The eggs should be large, ranging in size from about 65 grams for a pullet to 70-80 grams for an adult hen.
To have a deep dark red-brown coloured shell."
If your bird lays 4s at the beginning of the season and doesn't at the end, it doesn't mean she isn't a Marans. However; it's important to remember the standard of perfection calls for a deep dark red-brown egg and that is the goal.