Glenn Gardiner says:

There are a few things that can effect ballast color. The area of the country you model, not where you live necessarily. The type of track; mainline, siding, yard. The less maintained a track, the more likely the area between the tracks will be dirty and oil covered. Sidings and abandoned track can have little, to no ballast. And last, it is your layout, so choose whatever color you are satisfied with. After all, if you aren’t happy, the rest doesn’t matter. Glenn