I like the two peaks too.
I'm not sure if the idea is stupid or not, but if I need color variations of the same tree-population I reduce the object spacing to half, third part or smaller, I copy the population and then I play around with the diffuse color. Best results I get with also different scaling width and spacing variations.
Also you can change the connected object of the copied population if you find another similar but different looking tree in your library.
So I do it after all... I hope hereby I don't give you bad instructions