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

A few years ago, I touched up a few of my trains by painting their running boards white (and, in one case, black). Unfortunately, quite a few engines I applied this modification to got and still have a bit of white-out and black paint on their faces, and I'd like to cover those up with a paint identical to the one used on most of the engines' faces. Does anyone know of a good paint color to use?