Shaders are a familiar concept in the high-end Computer Graphics industry and are used in many other 3D applications. Fundamentally, a “shader” does just what the name implies – it “shades” something, a surface in your scene, where “shade” means literally “to apply colour to.” In computer graphics terminology, a “shader” is the set of instructions that dictate how a particular surface will be shaded, including aspects like diffuse colour, luminosity, specularity and reflectivity, transparency, displacement, and much more. Shaders often act on input from other elements of the scene such as light sources, determining the final colour of the surface by combining lighting effects with base surface properties.
The base colour is the main part of your landscape, you can eventually cover it up with surface shaders but you should usually pick your main colours here anyway.
Surface shaders - can be limited by Altitude, Slope and a lot of other things. We'll start with just Min Alt