Metallic things are either made out of metal or look like they are. Your metallic party dress might be covered in sparkly silver sequins. Yowza!
A metallic bicycle might be unpainted chrome — made of metal, in other words. Your metallic nail polish only resembles metal, glinting and shining gold. It's also possible to taste something that reminds you of metal and get a lingering metallic taste in your mouth. Metallic comes from metal, and both are rooted in the Greek metallon, "metal or ore."