One of the main attractions of the Marans breed (and please note they are 'Marans' after the town in France not 'Maran') is the dark chocolate brown colour of the eggs. As a utility bird, both the colour and the quantity of eggs are important and it is relatively easy to select for both of these things with a little care.
Without trap nesting (where a bird is ‘trapped' in the nest when she lays so her leg ring number can be noted), it is difficult to know who are the best layers in the flock. The way to overcome this is to hatch eggs that are laid very early and very late in the season. In order to lay more eggs in a year, hens will extend their laying season so the birds that are laying early and late eggs are usually your best layers in the flock.
Good Marans eggs are very dark brown in colour. Whilst the genes responsible for egg shell colour are very hard to isolate, it is known that dark brown eggs contain the brown pigment Protoporphyrin. The more that is present, the darker the egg.
Blue egg shells have a different gene at work. It is worth noting that these egg shells are blue all the way through to the inside. In Marans eggs, the pigment is deposited on the outside of the shell. You will notice if you scratch the outside of the shell, the colour ‘rubs' off.
To improve the colour of Marans eggs, you should only select the darkest brown eggs to hatch. Breeding from hens (and Cocks) that come from dark brown eggs themselves will ensure that you are concentrating this gene in your strain and get more of the same thing.
The darkest Marans eggs will be laid early on in the season towards the beginning of lay. It is still not completely understood why but hormone levels are believed to be responsible for this change. When a bird is coming up to a moult, her hormones are changing considerably and the change in shade of brown will be most noticeable.
Whilst you should be thinking about hatching dark, early season, eggs, it does not mean you can't hatch lighter shades over the course of the year - just select the darkest eggs that your hens are laying because although there is less Protoporphyrin deposited on the egg shells, it is all relative so providing you select the darkest eggs, you are still selecting the right eggs to hatch.
As with all breeds, I would recommend only hatching eggs of the right shape and the right size for the breed. There is a tendency for people to want to breed as quickly as possible from their birds and many breeds are being spoilt by people hatching the wrong shape of eggs. It is harder for chicks to hatch from the wrong shape eggs so hatchability goes down. By hatching from the wrong shape eggs, you are keeping the genes that determine this egg shape in your line so you are perpetuating the problem. When pullets come into lay, they will lay smaller 'pullet sized' eggs. Egg size will increase as they age but with slower growing large fowl, it is often necessary to wait until they are in their second season of lay to get the best size eggs.Personally, I find eggs from my 2nd year hens are bigger and better for hatching.