Hello Morfius,

The first thing that I would like to offer is that you start coloring the whole
thing first before you attempt to make a structure/texture on it.
Please start with removing the rectangular shape you have at the moment
it feels very unrealistic.
Select your light source. At the moment I think you want it on the top
left. Use the direction of the light source and your darkest color to
make a basis. Once you feel comfortable with the direction of the
shadow you can fill in the other areas with a middle tone.
Then you use a darker color between the darkest and the middle tone
to make some details here and there.
The highlights can be used at the end.
When you want grass in front of the mountains I suggest to use
a couple of different greens to make kind of planes between the foreground
and the background.

Good luck.

Best regards from

M.E.