Copper looks really gorgeous when it ages, it turns a gorgeous aqua/turquoise color, which is a beautiful contrast against the orange of the metal. Why not duplicate that pattern with paint?
Step 1: Brush on a layer of copper-colored paint. Let it dry.
Step 2: Using a wide flat-headed brush, dip the tip of the bristles in turquoise or aqua-colored paint. Apply over the copper in patches by touching just the tips of the bristles to the surface, dotting instead of brushing. This gives a more natural weathered look. Do this all over, wherever you want the patina look to appear.
I love painting trinket boxes. They are an excellent canvas for trying out new paint techniques. Plus they are great for storing thumbtacks and paperclips. Or really tiny treasures that you want to bury in your back yard.