Why is the background cloth of Keying green or blue?

Film shooting usually uses blue cloth or green cloth as the background, because in post-production, the silhouette of the specified color looks "transparent" and a specific background or animation is inserted in the "transparent" area. In theory, any color can use this function, and blue and green are not required. The higher the color purity, the more convenient it is to key the image. We use these two colors because we seldom use them in our daily life. Imagine if we use black as the background color, then the black clothes, black hair and even black eyes on the characters will be removed in the post-processing.