Very simple, any of these lines could be broken up by the surface shader. If you break one, only one of the grass stems will get the population variation through the surface layer and image map shader/painted shader or DandelO's additional idea with the merge shader. If you break two of them it will be more strong, because of two stems getting the color variation. All three would.. well, that speaks for itself.
You can break all by the same surface shader, and delete two default shaders. Then you start with one color stem. Or you can make three different surface shaders + three different image map shaders, for more variation, keeping all default shaders. Or use three different surface shaders + one image map shader + a color adjust, or you can adjust the color within the surface shader by the color itself or the luminosity or the degree of breakup. Lots of variation in the setup.
Is this explanation sufficient?
---Dune
