The way the egg shell is put together in the shell gland the basic shell goes first, either white or blue. The last half hour or so in the shell gland the hen puts a brown coating on top of that basic shell. If, for some reason, that egg is laid before all the brown pigment is applied the egg then the egg can be lighter than normal, whether brown or green. If it is laid really early it can pretty much look white or blue. The basic color is always there but shade can vary from day to day.
Something else that can happen. When a pullet starts laying or a hen starts laying after a molt her eggs are typically as dark as they will get. For many of them the longer into the egg laying cycle the lighter the eggs often get. I've had some hens that normally lay a light brown lay an almost white egg just before they start to molt.
Welcome to the amazing world of chickens and their eggs. It can be an education. Typically once they settle down you don't see a lot of regular variation. Some of us can tell which hen laid which egg just by looking at it. Many lay the same size, color, and shape of egg practically every day. You can still get a surprise though.
The egg making process is pretty complicated, there are a lot of parts. Some pullets get all that right to start with but occasionally you can get real excitement with some pullets. It takes them a while to work all the bugs out of that internal egg making factory. I don't know how long yours have been laying, they might settle down soon. In some ways that would be a shame, a lot less excitement.
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