I picked a charm that had one of the colors I wanted so I just used yellow paint on the silver parts. When I messed up or was a little sloppy I just used my finger or a tooth pick to clean up the paint while it was still wet. If you need to paint both colors just wait for the first color to dry and then proceed with the second color.
Once the paint is dry add a layer of Dimensional Magic Mod Podge to the top of the charm. If you have never used Dimensional Magic you’re in for a treat. And even without box of goodies from the company I would and have said this many times but DM it’s one of my all time favorite craft products. It adds a great resin like glaze that is crystal clear, has no odor and is super easy to use. Seriously you should check it out.
Next add a jump ring unless your charm comes with one. To open a jump ring you want to slide the sides out so they are parallel to each other as opposed to just opening it by spreading the ends apart. If you do that it messes up the shape and can break the jump ring.
Add a chain, throw on a white button up and cardigan and you’ve got an easy peasy Gryffindor student costume with a necklace that can be worn many times later.