As burnin said, it could be anything, like thin (single sided) leaves on one tree, vs thick (double sided) leaves on another tree. Try to apprach the issue more systematically - delete all the trees, exept for one copy of each that differs in colour, place them next to each other and render for longer (all that noise doesn't help to see what's going on). Render close-ups, do renders at different lighting conditions, try to see where from the perceived difference comes. If that won't help, try to turn off material's attributes one by one and see which one contributes to the difference.
