My first bit of advice for this is to answer the question of if the player will be able to get close to or on top of the mountains. If the player will always stay a fair distance away from the mountains, then I recommend using the terrain tool. Furthermore, I would recommend using some sort of height map to make the terrain. Just google for some sort of mountain height map, and then modify it to what you need. As for color textures, I don’t really have any particular advice on that, since I don’t quite know what you want for that. As for the fog, there are many ways you can make it so as to look like the image. I recommend you read about volumetric fog here:
You should also use it in combination with atmospheric fog;
