The models look nice. Regarding the color, it's difficult to get a proper opinion with the rendered view of 3dsmax.
Color palette in game would be the reference and still, I would be cautious as you will mainly focus on the colors / tone / saturation of the terrain mod which are sometimes not correct. I would advice to always check in game all your export, you could be surprised. A tiny change in 3dsmax could be much bigger in game.
Rendering a tree view should not take more than 5min even in old computer. The texture should be plain and not include alpha maps which (A. increase drastically the render timing, B. will make the coloring of your leaves even more complex). Better to have millions poly tree fully modeled than few thousands poly tree with alpha maps leaves.
Therefore, select well the models of your trees. Pines are the most difficult as the needles are rarely modeled but rendered with alpha maps.
Note: to speed tree rendering, select all your shapes, right click, object properties, check return black
