Probably not the problem but this is all I got. Did the color change over two weeks or once they were in your tank. Fish stores use bulbs to enhance colors and use certain backgrounds to make the color pop. Maybe they just look different under the daylight bulb that I assume you have.
They were a nice shade when I put them in the tank.
But, I seem to have figured out a possibility. When they are transported to/ kept in a store they are generally starved for a short while so they don't grow. But once there in a new tank with lots of food (tons in mine) they grow very quickly. Their shells don't exactly grow as fast as they do so the shell get thin temporarily. A couple of weeks and the shell should catch up and be thicker and a nice pearly blue.