Devil's advocate: while I agree that a big shiny black robot with a glowing red eye is decidedly NOT stealthy, there is something to be said for it being intimidating.
Indeed I think I read somewhere that the reason that the SAS use black fatigues in situations like the Iranian Embassy siege, is precisely because of the intimidation factor.
Despite what 70's ninja movies would have you believe, wearing black in the dark doesn't make you invisible. Far from it. In anything other than total pitch black (where the colour would be irrelevant anyway) black actually really stands out as a silhouette. You'd do better wearing desaturated shades of green & grey for blending in with shadows.