Ah, right. That's because the colour variation is applying the default colour according to it's particular patchiness gradient over the area. If the trees are using the default settings for the leaf texture the 'colour image' slider is probably set to '0.5', you could raise that to '1', or beyond, to bump up the colour.
Either try that or, you could raise the overall brightness of the colour variation shader, either in its own settings or with a new colour adjust shader following it. That'll mean the patches are brighter overall and will apply more of your leaf colour.
I also think the large scale inside the variation shader might be too large here for the near ground, I can only see a couple of patches where there seems to be a difference in colour over the closer area. Try halving the large constant scalar, I'd say. The thing with TG is, there's really no set rules and each scene can be entirely different, in terms of scale etc. It's not always easy to get a sense of what's going on by looking at a render but I'd say try that to begin with, the large patch gradient doesn't look to be having as much effect as it could, as it's being applied over too small an area, maybe...