Glass is a wonderful surface for your decoupage and altered artwork.  A metallic background can be achieved using Glass Medium, metallic powders and fine glitter.  Mix each in a one to one ratio, apply with a fine makeup sponge in thin layers, let each layer dry thoroughly.  When you use this method you avoid the brush marks you get with paints.  Remember to paint a final coat of acrylic paint and a sealer on to finish.  It's so easy to get a metallic looking on glass plates!

The project below was done using this technique....