When I built my first narrow gauge railway layout many, many years ago I used real soil from the garden - dried and sieved to add a more natural ground cover effect. With The Yellow Pelican layout I wanted to do the same and searched for a suitable red soil material.
I have used real red sandstone, found in Kidderminster (a natural rock outcrop at the side of the road) as the basis of this mix (see rock to the upper left).
The rock was crushed with a hammer and then sieved with a variety of different sized sieves to produce three different sizes of natural red ground cover; a very fine almost powder mix, a small rock size and some larger random rocks as seen above.
In addition I did the same with cat litter (on the left) and a lump of clay soil found in my own garden (on the right) these would be added to the red sandstone to vary the intensity of the red soil and add some natural variety.
Using all three coloured mixes and varying the grades of the sieved stones, I had a soil colour palette with which I could proceed. (This image also shows two of the three sample pots, I used to paint the layout).