The hen gets the pigment to color the egg shell brown from what she eats but also from her body. It depends some on her diet, but it is really common, pretty much expected actually, for a hen's eggs to gradually get lighter the longer she lays. She uses up that store of pigment as she lays. When a hen molts, she will recharge her store of pigment so when she starts laying again her eggs are really dark.
This is pretty obvious on a hen that has yellow skin. She will start out with yellow legs, a yellow vent, and yellow many other places. There is a specific sequence, but her yellow body parts will get lighter, practically turning to white, as she uses that pigment for her egg shells. When she molts, those body parts turn yellow again as she restores that pigment.
I have had some hens start out laying a medium dark brown egg but just before they molt, that egg has become really pale. It's not really white, especially if you have a true white egg to compare it to, but without something to compare them to, they can look really close to white.
There is nothing unusual or abnormal about this. This process is so gradual you don't notice it going on then all of a sudden you see what you think is a white egg.
Here is what the Egg Quality Handbook says about watery whites. This is written for commercial operations but a whole lot of this can apply to us. As you can see, watery whites can be caused by a lot of different things. Some typical causes are older hens, eggs stored a long time, especially in low humidity or high temperatures, or rough handling. But you can see what they say about all that.
http://www.thepoultrysite.com/publications/1/egg-quality-handbook/30/watery-whites
Good luck!!!
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