Yes, it can be done using the tint capabilities of Forest Color. The color range can be defined using a "Gradient" or a "Map", and this map can work in two modes: "Ramdom values" (a random pixel of the map is used for each item), or as "Texture", where the map is applied using the mapping coordinates of the Surface assigned in Forest.
There are multiple ways to use Forest Color in this scene. For example, using a colored map:
1) In the grass material, apply a Forest Color to the diffuse slot. If you was already using a grass texture, set it to the "Default" map slot.
2) Turn on "Override", Range 100% to 100% (or less if you want to reduce the tint's strength), "Map->As Texture on Surface".
3) Assign the colored map to the "Map" slot. It will use the mapping coordinates of the Forest's surface (aways from map channel 1).
4) Set "Blending Mode" to "Color" (the default value). This parameter defines how the tint is blended with the base color. It works in the same way that the "Composite" texture map, or the Photoshop color modes.
This mode is appropiate if we like to use a tint map with multiple colors, not only a variation between two tones (yellow/brown). But also it is possible to use Forest Color as a mix map, to blend two materials (as you describe for this scene). For example, using VRayBlendMtl:
1) Apply Forest Color to the "Blend amount" map slot.
2) Turn on "Override", Range 100% to 100% (or other values), "Map->As Texture on Surface".
3) Assign the white/black map to the "Map" slot.
4) Change the "Blending Mode" to "Normal". This mode instead of tinting the "Default" color, replaces it by the result of the map.
I have attached a sample scene using this method (Max 2010, VRay 2.0).
PD: in this example i have used the Tint parameters of Forest Color, but you can find them duplicated in the Forest object (Material rollout). If you define them in Forest (applying a non-zero range value), the Tint values are applied to all the instances of Forest Color (except those using the "Override" checkbox).