Strange. I can only imagine things happening to leaf color if variation is linked to a distance shader attached to the render camera (which would not be a wise thing to do, and is probably not the case), or reflection is changed by changing of the frustrum (suddenly reflecting detail that isn't inside the 'scope' of the frustrum, so to say). But that may be too far fetched as well. And I don't use the built in color variation method, so I don't know if that could cause it.
I believe there was an issue like changing leaf color in another thread a few weeks ago, but cannot find it right now.