There is no strong reason to believe that metallic dragon scales are actually made of metal rather than just colored that way. In the absence of a specific statement that dragon scales are in fact made out of the metals they resemble, the default position should be that they’re no more metal than red or white dragon scales are.
Metallic coloration is relatively common in real-life fish, insects, and even some reptiles*; and the Monster Manual sections for the different metallic dragons often talk about how the dragon’s scales start out dull as a wyrmling and only develop a metallic sheen after many decades or centuries of life. This slow change suggests it’s mere coloration.
*The silvery color associated with fish scales and some insects is actually crystals of the amino acid guanine, a completely non-metallic substance!