**Copy the shader you used for your terrain onto your grass, but use "object" texture mode instead**

Terrain shader:
[![enter image description here][1]][1]
Grass shader:
[![enter image description here][2]][2]
Note that on the grass shader, the "object" texture coordinates are used, and the base terrain is used as the object.
**You must select the terrain (in my case, "GridGrass") as the object**

[![enter image description here][3]][3]

**Terrain texture transformations must also be made on the grass shader.** In the example above, a "mix" node is being used instead of a "mapping" node.

Final result
[![enter image description here][4]][4]

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.

  [1]: https://i.sstatic.net/EpwdW.png
  [2]: https://i.sstatic.net/M3I0a.png
  [3]: https://i.sstatic.net/PJp1N.png
  [4]: https://i.sstatic.net/T4Puc.jpg