Quoted from fosaisu:
Maybe they can talk to Stan Fukuoka about it, it's a little hard to believe the grey was anything other than a placeholder (especially since the gangsters' suits are just a different shade of grey, though maybe those were meant to be colored differently as well). Consulting Stan on those color decisions would allow them to keep an "authenticity" to the remake while substantially improving the look of the cab sides.
I love the grey. To me, it looks like a page out of a graphic novel and the color fits the feel of the mobster scene.