How do I set the MaterialPropertyBlock for terrain grass?

Hello,

I want to apply a replacement shader to the terrain grass and change the material properties using MaterialPropertyBlock. I've tested the SetSplatMaterialPropertyBlock function as the following code

 var mpb = new MaterialPropertyBlock();
 mpb.SetColor("_CustomColor", terrainColor);
 terrain.SetSplatMaterialPropertyBlock(mpb);

The replacement shader works fine for terrain and grass. But, the _CustomColor only applies to the terrain but not grass. How do I set the MaterialPropertyBlock for terrain grass?