Bolters, 'coloured' or metallic?

 After discussion with my good friend 'painter john' he queried why all my marines have metallic bolters. To me its simple, they are made of metal. A lot of the bolters on the interweb and in codicies etc show them with coloured casings which I have never been a fan of, John suggested painting them black, with a grey highlight to pick out the edges.
 
Am still not decided, need to ponder on it a bit longer...