It's not exactly a per instance difference, but with a careful mask and some fiddling you might get a tree by tree difference. It's certainly not within one instance, that's not too hard. For now its just color patches over the whole pop, but they are easily changeable (enlarged, made stronger...). I am now rendering a sample where the lower half of one population of trees has a autumn-like color gradation, higher up the same pop is covered by fresh snow, and real high up the same pop has dropped its leaves... still needs some time.
I have tried every angle to not have to use an image map, and do it procedural, but to no avail.
---Dune