UV Map, change texture color (lighter/darker)

I'm trying to figure out how I could make my own lighting system for my UV map in Unity. I have the theory nailed down (I believe) but I'm having problems on the practical side. I have a voxel (minecraft-like) map object that is generated at runtime. Let's say the UV map for a face is done as such: UVm.Add(new Vector2 (0, 1)); // 1 UVm.Add(new Vector2 (1, 1)); // 2 UVm.Add(new Vector2 (1, 0)); // 3 UVm.Add(new Vector2 (0, 0)); // 4 How could I go ahead and make the texture on that particular face lighter or darker?