Green and purple stars do exist. The color of stars depends on their temperatures, and they emit radiation throughout the visible spectrum. But when a star emits peak radiation at a wavelength we define as green, it also emits radiation over the rest of the spectrum.

Although you can spot many colors of stars in the night sky, purple and green stars aren't seen because of the way humans perceive visible light.

Pisaster ochraceus, generally known as the purple sea star, ochre sea star, or ochre starfish, is a common seastar found among the waters of the Pacific Ocean. Identified as a keystone species, P. ochraceus is considered an important indicator for the health of the intertidal zone.