I should have also mentioned the advice I was given. Once painted the contrast paint isn't as hardwearing as their others so the advice was to give a spray with matt varnish.
Also if you want funky coloured metallics that harken back to the late 80s/early90s paint ranges he suggested undercoating with a leadblecher spray followed by a good drybrush of necron compound. The drybrushing is aparrently required to give some texture for the contrast paint to stick to because the leadbelcher spray is too glossy.