I also poured some roadways from lightweight hydrocal mixed with flat black paint diluted with an equal part of water. That yielded a nice gray color for the road. However, I used diluted matte medium to fix the ground foam on the adjacent hard shell. I didn't protect the road from the overspray and the moisture really ate up the hydrocal road. I ended up mixing up more hydrocal to "repave" the road.

I used double-sided mounting tape to define the edges of the road, poured a soupy mix of hydrocal into these "molds" and smoothed off the top with a wide putty knife. At the grade crossings I simply poured the hydrocal right over the track and smoothed it off. After the hydrocal had started setting I used the corner of the putty knife to clean out the flangeways, then ran an old freight truck down the rails through the damp hydrocal to insure clearance once it dried completely.