Why aren't plants black? 🌿

I know I'm way late to this, but I own a number of planted aquariums. Red plants are some of the most challenging to grow because if you don't give them enough light they turn green. We usually say that the "red" in plants is essentially a tan that it uses to keep from getting burned because you have to saturate them consistently to keep them red. Having enough iron and CO² are also factors but by far the light intensity is the most important. I wonder how that would factor into those theories...