Why aren't plants black? 🌿

In AP Biology courses they tech us that evolution and natural selection can only act upon existing structures (this is why animal have vestigial structures like whales having hips). Plants evolved first in the oceans. And thus they evolved to be best fit to photosynthesize under water. Water absorbs color light mostly in the spectrums of red, yellow and green, leaving blue and purple behind to be absorbed for energy. These are also the wavelengths of color light that plants use primarily for photosynthesis, along with the lower end of the spectrum red and oranges. There was not much green in the oceans for early "plants" to absorb so they ended up reflecting them. As evolution progressed and plants adapted to land life, they kept these characteristics, unable to sprout new ones from genes that don't exist.