Copy the shader you used for your terrain onto your grass, but use "object" texture mode instead
Final result:
Terrain shader:
Grass shader:
While you could choose to apply the terrain texture directly, I chose to multiply the original grass texture with the terrain texture. This way, the grass retained it's colour but would be lighter where the terrain is lighter and darker where the terrain is darker, as seen in the image above.
The best way to experiment is to make a plane and use your grass shader on the plane to check it matches up with your terrain. I had to tweak the texture coordinates quite a bit.
Before:
After:
