Well, I've even had troubles with those trees as well.
I did a test of the trees on the default planet and it looked ok, the colors were dark and a little washed out, but you could defiantly tell that they were green.
I know it's not my atmosphere, I have the samples up to 128 and the default 16 on the default sphere looked ok, so I don't know what's going on here, these trees are on high quality.