Thanks for the file.
In this scene, since the map is the same for all instances of Forest Color, you can switch off "Override" and assign the texture directly at Forest->Material parameters. Once adjusted the range (100% to 100%), the map is visible on viewport.
Regarding Blending Mode, you can use "Normal" with a white base color (i.e. without grass textures), or set "Color" mode. Both generates a similar result.
Note: there are tons of items generated on this scene. To optimize it, you would use a more dense map (even a "Full") or lower Distribution->Threshold, while increasing X/Y Units. You can find more information about these parameters
here.