The color of Kryptonite that is not harmful to Superman is green Kryptonite. In fact, green Kryptonite is the most common and well-known form of Kryptonite that is found on Earth. Although green Kryptonite weakens Superman and makes him vulnerable, it does not kill him. However, other colors of Kryptonite that are harmful to Superman include red, blue, gold, white, and black Kryptonite.