How to modify output color of Built-in Terrain Shader with more than 4 Splat Textures?

I changed the Built-in Sureface Shader to modify the output color in certain circumstances. In fact I implemented Fog of War for parts of the terrain that isn't visible by the player. That works great as long as the terrain is using 4 splat textures. In case it's using more the 4 textures, the color modification is only applied to the first 4 textures. The other textures are rendered normally, as though nothing had happened. I tried to modify the color using the actual surface function (*SurfaceOutput.Albedo*) and tried modify it using the (*finalcolor*) function. Both worked for the first 4 textures. But the the other textures it doesn't work. Any Ideas?