Everything is in title. It has for consequences that when other trees than oaks or darklands trees for darklands generate them in these biomes, they keep their colors, and it makes kinda strange.

You will need to be more specific. Out of the vanilla trees, all leaf types (except Spruce and Birch, whose colors are hardcoded into the game) change colors inside AbyssalCraft biomes that apply different foliage colors.

I spoke about these trees. ^^'

Since it's vanilla mechanics, there isn't much I can do about it (as I don't plan on rewriting base game behavior).