I was told by a biological dentist, and another regular dentist, that dental composite manufacturers use a very small amount of radiactive something (thallium is a good search) to make the composite show up on x-ray. Neither cadmium or mercury are FDA approved pigments, if you meant to imply that. If they were manufacturers would use them in cherrios and candy.

Probably something along the lines of red#2 or so as those types of pigments are labeled. Dyes. Hopefully vegetable based, some animal based.

However, dental materials are not really subject to FDA approval because they are not a food. which is why mercury can be found in amalgam, cadmium in dentures, and titianium and nickel in implant prosthesis devices, etc.