Laws in the US require all cars have a reversing camera. That in turn requires all cars have a colour screen. That's quite expensive, but necessary to meet the law.
When you already have a screen, putting car controls on it costs basically nothing.
I doubt a black and white screen would meet the law - they're terrible at showing photographic content. A greyscale screen might, but a greyscale screen isn't really a standard component in the 4+ inch size required to meet the law and likely wouldn't be any cheaper than a color screen. The law isn't super precise about resolution etc, but if you can't clearly see a dummy child while glancing down, then it is illegal.
When you already have a screen, putting car controls on it costs basically nothing.