Gus is correct about washes not being suitable for use on foam. My layout landforms are mostly patching plaster over screen, with some areas simply bare plywood. I went to Walmart and bought a gallon of their cheapest flat interior latex - I picked a "dirt" colour from their samples.

I thinned it just slightly for the plywood areas, to make it easier to spread, then thinned the rest about 50/50 with water. I applied that with a 2" "throw-away" brush, more-or-less just slopping it on the plaster. The whole process was very quick and easy, and really changed the look of the layout - from snowy landscape to desert. I was surprised by how much the whole room was darkened, though,

and I have
lots of light fixtures in the layout room.
Here's what the plaster looks like after the application of the wash (this area will eventually be covered with trees):
Here's a look at another area after some ground cover has been applied (after the wash was dry - it doesn't stay wet long enough to allow the ground foam to stick to it):
Here's an area that is mostly right on the plywood, including the track. I didn't apply any scenic material while the paint was wet - by the time the painting was done, most of it had dried anyway.
For the foam areas on your layout, use a suitable brush to apply the unthinned paint - I simply painted up to the tie ends where track was already in place - the ballast will cover the foam that shows between the ties.
Wayne