(sorry by the posting issues, we made some wrong changes in the board's permissions. It must be fixed now)
Random Tint step guide:
1.- Convert your materials to "Forest Material". It works as a generic material container, so you may put in "Base Material" your VRay materials or whatever other types.
2.- On Multi-sub materials, always use "Forest Material" as a sub-material, on this way:
3.- Enable random tint in the Forest object, setting some value to the range parameters (Material rollout). A range of 50% -100% will assign a randomly percentage of tint from 50% to 100% for each tree.
4.- Adjust the color in the Gradient, or use a map with the available options.
You also can adjust the tint parameters individually for each material, enabling the "Override" checkbox in Forest Material. As instance, to disable tint for a "trunk" material in a multi-sub, or adjust the color for the leaves.
If you need a sample scene or something else, please tell me.