Can porcelain crown color be fixed if crown hasn't been permanently bonded to tooth? (Photo)

Hi:Most likely the color can be fixed.  It’s best not to let the crown stay in your mouth for too long before it is removed and the color is corrected.Best,Dr. Maddahi

Your all porcelain crowns are translucent more so than your bridge. That is why you are getting the gray appearance. It is possible to potentially stain the crowns to make them lighter but will be very difficult. If you have any chance, it will have to be a custom shade by the lab technician. They could do this while you wait if you can go to the lab. Your dentist may have to remake them if his/her lab cannot accomplish a custom shade. Yours is a difficult case but can be accomplished if you can get a custom shade with the lab. Don't settle.

Its great you're getting to "test drive" them before they go in with final cement.  Depending on what type of material the crowns are made of, the color of the porcelain can be influenced by the color of the cement that will be used to bond them in with.  Typically, there are try in pastes that can be used before to check if the color can be corrected without having to send the restorations back to the lab.  If that is not enough, the lab can correct the color of the porcelain without having to remake them, but again depending on what type of material they are made out of.  Best of luck to you.