The color green has long been symbolically associated with environmentalist political causes, because the earliest environmentalist concerns were about literally-green plants being replaced by human-made machinery that is typically brown and grey. The fact that, today, the biggest environmentalist political concern is atmospheric CO2, and the fact that one effect of larger amounts of atmospheric CO2 is literally more green, photosynthesizing plant life, is a true enough fact; but not really relevant to the color symbolism.