I'd go for black - nice and understated but still mean and menacing. The sheer size of the things says it all. I think silver will just blend in with the discs.
If you haven't got the anti vibration plate for them you can get this from the dealer, it has the S4 logo on it aparantley. I've been in two minds about doing this, so I think I'll just pain them black and miss out the decals. Whatever you go for, they are definately going to look aggresive.