My starting technique:
1. Design my displacement in Photoshop
2. Apply the displacement to the number of boards in my route
3. Apply the color AJS Grass 06 in smallest scale to every board (unless there are boards with ocean) as a BASE COLOR.
4. Add water to rivers, streams, lakes, etc.
5. Color in the edges and bottom of said rivers, streams, lakes, etc.
6. Begin laying track, then roads and the rest of the scenery.
Note: The reason I use the lowest scale of AJS Grass 06 on all my boards is that when laying track, roads, fences, etc. there are straight patterns in the color which are visible in Surveyor but not noticeable when in Driver in the cab of a locomotive. The straight patterns can be useful for laying nice straight sections of track, roads, and fences.
This BASE texture of AJS Grass 06 gives the route a nice green "feel" to the scenery. As I go about designing the route, I use other colors to enhance different areas, such as crop fields, shadows under forests, cliff sides, rocks, ballast, concreted areas, the gravel usually found on each side of the road, etc.
Mine is just one method. I'm sure there are many more!
Dave