We already know why all plants appear green: it's because they're full of the light-absorbing chemical known as chlorophyll. But since they appears green—bouncing back green and yellow light waves—it means it's not 100 percent efficient at absorbing all of the sun's rays. So why aren't all plants black instead?
It turns out that scientists aren't quite sure just yet. The leading theory is that modern plant life evolved from bacteria that lived under the sea and only had access to blue and red light streaming through the deep water. So they learned to survive with what they had, and came to rely on the chlorophyl molecule which was adept at turning red and blue light waves into usable energy. And that chlorophyl actually gives unique advantages to plants that we're only just starting to discover.