Colorless summary: A gray Earth, where humanity was transformed into deformed beings. The curtains open on an action series where the goal is to recover a lost world, with the help of a strange-looking gun! An enormous solar flare caused the planet to lose all of its colors. Humans are deformed and declining in numbers. The key to reclaiming the old world lies within the power hidden in 「color」, and one 「girl」. Knowing that the few remnants of colors in the world yield a great power, Avidia, who is conducting research while protecting a girl named Chie, uses the colors to expand his own power, and throws himself in a battle against the Cult.