Star did not become black and red until after his power was fully realized. His power is nothing short of god-like, which allows him to manipulate many of the natural properties of the Pokemon dimension and to combat Arceus' tyranny.
Although Star was physically born as a Staryu, he has actually existed since the creation of the Pokemon dimension. Born from the same chaos (Rage) that Arceus was born from, he was cast down by his "brother" and stripped of his powers so that Arceus could rule alone. He rose to fame as a renowned fighter along with his group of childhood friends, which began to intimidate Arceus. Arceus orchestrated the destruction of his home villiage, and had one of his agents kill one of Star's closest friends. Rage then revealed himself to Star, unlocking his restricted powers and guiding him on a long journey to defeat Arceus. Though he hasn't killed Arceus as of yet, Star has become a fully realized god within the dimension and has forced Arceus into hiding. He is usually able to enjoy a peaceful life with his wife, but is occasionally forced to quell an Arceus uprising. He left control of his army to Stark, so he could pursue more dangerous tasks alone.