I think I know what you are talking about as regards paintings but have not seen the effect in a photograph. There is "metallic" RA4 colour paper around today that actually contains no metal at all but which is much favoured by photographers to impart a metallic-like sheen particularly to brightly coloured objects such as cars. Looks good on some portraits too. You can get prints on this paper and on aluminium as well as various acrylic substrates from just about any prolab. OzJohn