How does one approach drawing mountains for landscape/environment concept art
I don't have any experience with Photoshop textures, so I'll limit my answer to general information. 

Colors

First pick the "pure" color of the rock in bright daylight, which will be the base color of your mountain. In the first reference image of your question, that's a dark grey with a tint of brown. For other types of rock (like sandstone or marble) the base color can be a very different one.

Then adapt the color according to where the sun is situated in your picture:


The mountainside facing the sun reflects the yellow sunlight. Add more yellow or light brown to the color on this side
The mountainside opposite of the sun reflects the blue sky. Add more light or dark blue to the color on this side.
The sides perpendicular to the sun (neither facing the sun directly, nor facing away from it) should be painted in the base color.
If you want to paint a mountain at sunset, the sky is darker and the sun is red instead of yellow, so add red to the base color for the sunny side and dark blue to the shaded side.
When painting snow, the base color of snow is pale blue or grey. only the snow facing the sun is bright white.


Perspective

The common way to add perspective to a mountain range is to put a thin veil of sky color behind each mountain. That means:


The rock base color for the mountain closest to you is unchanged
For the mountain behind that, add a little light blue to the base color, then further mix this adapted base color as described above.
For the next mountain behind that, add even more blue to the base color
If the sky has a different color (like sunset), add this color to the base color instead of blue
If you want to paint a foggy / misty scene, add light grey or white to the base color (your 8th reference image with the horned spires shows this well)


Traditionally, you would start painting from the background to the foreground. When working with Photoshop, you can add a new layer, stick with the same base color and add a semi-transparent veil on top of the layer.

Bare Rock Texture

This becomes relevant at a closer perspective than the general chunky outline of a mountain. It determines how you fill in the outline with a pattern that looks natural.

Here you need to imagine how the mountain was formed during millions of years. Reference images from different places in the world may give you a feeling for how rock corrodes in different ways.


Is it a fold mountain? Then there should be regular, reoccuring lines running in parallel (as shown in the snow deposits in your first reference). These lines can be at any angle you choose and even curved (as in this reference).
Is it a fault block mountain? Then there should be high cliffs with vertical fracture lines. There can also be horizontal deposit lines in different base colors, depending on the different kinds of soil that formed the mountain.
Is it a volcanic mountain? Then the lines should all originate from a central point slightly above the top and spread out towards the base, reflecting how the volcano expelled lava and how corroding rocks and ash deposits slowly slide from the top down to the base.


Other "Stuff"

A mountain is never just a structure of bare, clean swept rocks. There are deposits of soil, snow, ash or other things accumulating on the rock surface. There are plants growing on this soil. All this adds a rounded slope to the area and another color.

You can purposefully leave this stuff out to create a more jagged, dramatic landscape or you can add more of it to give the landscape a wheathered and rounded off look.

These kinds of deposits tend to be few on the top of a mountain, filling occasional gaps at a steep angle. Towards the base they become more and filled with more material, resulting in flat angles and round outlines.