You can not mix different materials for the terrain but you can replace the shader that the terrain uses to render.
In the Terrain Settings (the gear symbol on the right) you can select the material that the terrain should use.
Keep in mind that for the terrain to function normally you need to select a material that has a specialized terrain shader selected.
if you need to tweek the terrain shader you can get a copy of it in the build in shader bundle.
First of you need to write or buy a custom shader like @fffMaizbier said then head over to Terrain
Then head over to your terrain shader material and play around with the sliders from the shader.