I noticed mine after eating algea or moss will look green and if they managed to snatch a lot for wafers from my corydoras and goldfish, it will will look brownish. Reason being they are translucent and I can tell if they have eaten their food.
I also, noticed that once they adapt to the water parameters, they will start to shed their whole shell every few weeks. So not sure if it is due to these reason that you are noticing colour changes.
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