Hi there - don't judge the colour of the eggs she is going to lay until she has laid at least 20 eggs. The first ones can sometimes be covered with a calcium "veil" but after around a couple of doz eggs then they will probably lay as dark eggs as they will produce during their lifetime.
Even when they laying tract is fully established the shell colour will vary slightly day to day. All hens lay their eggs in what is known as clutches ie a modern hybrid may lay as many as nine or more eggs before having a day off. A more traditional breed may only lay a couple then have a break, and a really poor layer (some French Marans can be fickle layers) may lay an egg or two then have a week or so off.
With a Marans the shell is white and the colour painted on, so if there has been a day (or more) without an egg the pigment will have had time to build up and the first egg of the clutch will be laid with a darker colour shell
One of the problems with the cuckoo gene is that it is dominant, so pretty well anything bred to a cuckoo feathered male will tend to give cuckoo coloured pullets with a Marans look. In order to increase egg numbers many times in the past a male hatched from a dark egg has been put to high egg producing females, and the resulting chicks look similar to cuckoo Marans but will not lay the darker eggs.
There is information on The Marans Club Website, and quite a lot of detail on my own website.
All the best
Sue