Part 4 opens with an explanation that life adapts and evolves to meet the challenges of an environment, though the chapter is, of course, written far more artistically than that.
The algae and lichens created by the Underhill terraformers are no exception. Their DNA alters, providing them with new abilities to meet the challenges of life. Some strands die out; others thrive; still others consume the dead and the weak.
And as life changes, the conditions that changed life also change as a result of that life. Think of it as a circle of life. Or maybe a yinyang of life.
In the form of algae and lichen, the colors green, gray, yellow, and black have come to the Martian landscape.