The idea that the red color exists specifically as a warning to predators does not hold up to the basics of how these particular species live. Water mites have little globe-shaped organs dotting their bodies, called glandularia, each with a little opening and a little bristly hair called a seta. When something, say a fish, brushes up against that hair, it triggers the glandularia into releasing a milky fluid into the water where it mixes into a tackier more viscous substance. So when the scientists saw in the context of their own experiments that fish seemed to quickly stop trying to eat the water mites, they decided that the rest coloring was water mite's way of telling fish, "we're the sticky prey." The likelihood that they would have evolved this trait for the purpose of warning fish is low. It's more likely that the pigments are a way of protecting them from ultraviolet light. It just maybe happening to also protect them from fish as well.