@bob76 . This bothered me greatly as well! We were up at the actual track last weekend and everything else on this track is done in such accurate detail, I couldn't understand why they would use dark trees with big green leaves, it just didn't look correct at all. Then I crashed on the esses and noticed that the lone tree there when close was a very good resemblance to real life. Then I noticed the trees changing (from dark to light) as I got closer to each one. Then the issue bothered me even more.
The issue 100% for me was the setting "FOLIAGE LOD QUALITY". This is under OPTIONS->SYSTEM->VIDEO->ADVANCED->FOLIAGE LOD QUALITY. For me at Bathurst I like at least a setting of HIGH. My original setting was VERY LOW, which gave me the dark trees thing.
I fooled around a bit with the BASIC setting of FOLIAGE, but I can leave it on LOW and its OK for me.
Hope this helps? and I hope your PC can handle the LOD quality setting change?