Hey there!
For testing purposes, it may be good to create a material that is just Green into the base color and assign this to your trees temporarily. If a green only material also change color, the problem would somehow be caused by your scene, lighting, or settings. If this green material does not change colors like your trees, the brown color is coming from your materials.
One suspicion is that the brown is actually your bark color blurring into your green branch when the LODs change. If this is the case, one solution would be to create your own LOD materials/textures that are all tinted to match the green of the branch rather than the brown of the bark.