Plants are green(On earth) because they reflect the green color (which the sun spectrum is strongest) because they have probably adapted to better regulate different lightning condition during the day, for this they use two different pigments (chlorophyll a(blue), chlorophyll b(red); or other pairs)
Outside of human-directed breeding, we typically only see darkly colored plants in locations with low light and with few other evolutionary pressures: deep understory tropical plants, for example.
Meanwhile, in locations with high light and other evolutionary pressures, plants can sometimes evolve to be quite pale. For example, desert succulents are often very light colors because they need to deflect light to keep cool and preserve water.
Simple example: “C4 carbon fixation”, which allows photosynthesis to proceed with much less water available, only evolved ~30 million years ago. This was the revolution that allowed modern grasses to emerge, and thus changed enormous barren deserts into the savannahs that early humans made their home. Despite being such an improvement, this is not yet a “standard”; trees, for example, still use the “old” inefficient “C3 carbon fixation”.
Despite photosynthesis having been around for hundreds of millions of years, fundamental improvements still happened just 30 million years ago. I would suspect that there is more that can still be improved.