How do I set shader settings for a terrain?

I have large terrains with a heightmap and texture map applied. I understand the terrain has a simple shader, but how do I set the values of the terrain? Right now my terrains are displaying to bright and I'd like to darken them.

From what i understand: you can't change the material of the terrain, only the textures. Use the paint button and start texturing. If you want it only in one color make a solid color texture.