Metallic salts are metallic compounds used in nearly all standard store-bought hair dyes. They're sold as "progressive" hair dyes that blend and "look more natural over time." These dyes are metallic-based. The colors are produced by a reaction between metallic salts and the sulfur in your hair protein. The metallic salts interact with the sulfur in the hair protein(keratin) and then turns the protein to the engineered shade.
Here's the problem: metallic salts, aside from being chemical manipulators, do not mix well with any other chemicals. If you color your hair with a metallic-based dye, you will be limited on changing your color or adding deep treatments. The adverse reactions to other chemicals could result in hair chemically smoking/burning and potentially melting off. And these added bonuses: dry out the hair, damage the follicles, dull the sheen, and make the hair become overall brittle. Just stay away.