It isn't the colour, it's the texture.
You need to get the specularity right, and then select one of the metallic preset micro polishes. Make sure you set an environment for it to reflect off, so that it looks like metal.
For extra realism, you could scan in a bump map of pitted metal or something.
The graininess can better be sorted out in Photoshop. Have a look at the System colours to find out which colours have a lot of variations. Some (such as yellows) work very poorly in System colours.
PS - does anybody know what caustics actually do?
((Bonus PPS, for those who don't know, the quote used in Pulp Fiction (Jules' sig) isn't actually in the Bible at all. I'm not sure what Tarrantino is trying to say by that ))
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M A R T I N T U R N E R