The best way to paint coloured metallic is to buy coloured metallic!
Don't waste your time with silver and washes unless you're simply going for tinted silver. If you actually want a coloured metallic, just buy a coloured metallic.
Testors sells some coloured metallics, I've personally used Aqueous Hobby Colour. I think Tamiya makes some coloured metallics. Go to a model aircraft/car hobby store and they often carry a range of metallics.
This was done with Aqueous Hobby Colour, which are acrylics. If you're willing to try enamels, there's heaps more options as well.