how do i turn silver coin to rainbow color?

Heat place it in an oven.
Sulfur is the active toning agent most of the time.

Sometimes just wrapping it in paper and leaving it on a desk for a few months does it.

Keep the coin over flame. Several layers of silver oxide form on the silver coin. These layers can disperse the sun light into VIBGYOR.

chemically you cant but physically just colour it.

Use dye.