They should probably stick with grey. Grey says "nothing to see here, it's just grey, nothing interesting, move along, not worth your time wondering about this part of the pie, which, if anyone took this much of an actual pie you would think he was a pig, but nevermind that" without anyone noticing that that is what it is actually saying.
Any other color other than grey might stimulate retina reflexes in brainwashed voters, thereby triggering an initial thought, perhaps nothing more than "huh?" but "huh?" is very very dangerous as it might lead to further inquiry and that is completely unacceptable.