I think it's fair to say that silver is common for MBs generally - especially AMGs - and black is up there too.
To be different on an AMG you really need to be opting for not silver and not black - green, blue, etc. The red C32 AMG that did the rounds looks stunning, albeit different, and will have a smaller market than a more popular colour.
Silver is probably popular for AMGs partly because it's the MB colour (ie no paint on racers), partly because it's used in the AMG brochures, and partly because it's the colour that shows the extra definition on the AMG body kit most clearly. It's far easier to spot the AMG kit on a silver car than another colour for this reason.