The big key with how clams appear to the eye is as follows.
If it looks blue from above, it will look purple thru the side of the tank.
If it looks green from above it will look blue thru the side.
Clams are almost always photoed from top down, that way they look their best, and show the most detail of the clam.
Lighting has a factor to play with how they appear as well.
When I was moving into my home, we had some clams in a bucket of saltwater sitting in the sunlight, you would not believe the difference. Natural sunlight pounding down on top of clams are beautiful.
Climb up above your tank and look down on it, with the water circulation turned down or off, and I bet it will look just like your picture from the store.
Clams are fantastic, Rob