Why don't you just use the real stuff you can get outside for free?
I use a lot of real dirt on my layout, not just dirt roads, but for general scenery.
Just get a flower sifter and get all big chunks out. Then spread some white glue down where you want your road to be, and sprinkle in the dirt.
I did this for my farm dirt roads here.
check out this photo there is an area of loose dirt between the tracks then a tarmac road, more tracks then a gravel road.
For the loose dirt I just slapped down some varnish sprinkled some sieved dry dirt from the garden on it then once the varnish had set I wet the surface then soaked it with 50/50, PVA glue/water to seal it.
The tarmac and gravel road are built by.......
1; laying down the shape with coloured plaster of paris then when its still semi-soft sprinkling it with fine sand based balast and rolling it down.
2; As sood as it becomes firm I wet the surface with a paint mix containing
1-2 parts water based paint, 1 part PVA glue, 3 parts water and a dribble of dish detergent
3; I then sprinkle the wet paint with more balast and lightly roll it down. If you dont roll it enough the surface wont look hard packed,if you roll it too much it will start lifting on the roller.
4; Repeat stage 2 and 3 45 minutes later if the surface isnt perfect.
5; A couple hours later once its fully dry soak the surface with a final coat of the paint mix
For the tarmac I used 2 parts black paint. for the gravel road it was 1 part light brown paint. I were very pleased with the results.
The coloured plaser of paris is just plaster of paris with a small amount of pva glue to stop it cracking and a bit of water based paint mixed together.