We are doing a book study of Bernie Berlin's book "Artist Trading Card Workshop" on the NGS group. This week the section is "Metal Tape". Where I live, you can get this in the hardware store called "Flashing Tape". There are many different ways of using metal tape. One technique Bernie shows in her book is to color it with alcohol ink and tear up the tape . First drop several colors of alcohol ink on to the metal tape with a few drops of blending solution. Use a heat gun to spread and set the inks.
Add additional drops of ink as desired. Tear metal tape into smaller pieces and layer on to the card back. Use a brayer to smooth the pieces down as you go.