Any idea how to achieve the grayish-white hue of most of the engines' faces?

You most likely didn't protect/remove the shells before painting the running boards, and that must be why the paint got on the faces. As suggested by TM2K1, it might be better to just repaint them entirely. If you can't get a good mixture, I'd suggest alcohol to remove the black and acetone or some other solvent for the correction fluid. Just clean the faces if needed first and make sure not to wipe the features off. Also, you should use masking tape to protect the parts that should still be red, black, etc. and be careful if painting handrails and boiler bands. I learnt that the hard way with engines like my original Edward.