When variations like this are common, I suspect that color isn’t important. Orange starfish aren’t more likely to be eaten than purple ones or more likely to find mussels to eat so natural selection doesn’t favor one color over the other. The only thing we can really say is that the different colors exist because they can.
In many other cases, one color morph is standard and the others are rare, such as the white and pink morphs of normally blue chicory (Cichorium intybus). In this case there probably is strong selection against the odd colors because pollinators use the color to find the flowers. If bees learn that blue chicory flowers are good, they go from blue flower to blue flower and tend to ignore the white and pink flowers, which therefore don’t set as much seed.
It’s also possible that the chemical differences that cause the colors we see also cause some other difference that is important to survival and reproduction and color itself is just a side effect.