The cushion moss is certainly very green, but you’ll note that the grass and moss on Skellig, your inspiration island is distinctly bright green as well so I’d hesitate before writing it off completely.
You could alternatively try swapping the bog blocks with grass and trying some different biome mixes. Jungle might produce the kind of dark green you’re after. If you swap in the grass try //replace grass $Jungle then warp away and back to see how the new biome colors look.
If there’s still too much green it may be worth swapping out some of the grass blocks for more northern pebbles/terrain set stone.
My internet is broken for another day or so but experiment with those a bit in the meantime and see how they look.