What paint to use for faces?

I simply mix white with a (really) tiny amount * of black, which I find sufficient for the "vehicle face" colour in most cases.

* something like the very end of a Q-tip, if not ever so slightly bigger. If you don't repaint the entire faces, brighten or darken if necessary until you feel you can barely see the corrections.
 

No idea, we might have

Yes, we did: https://blueplastictracks.com/showthread.php?tid=4075