WebMD states that vitiligo and pityriasis alba cause light patches on the skin. While people of any color can have these two conditions, it is most easily seen on dark skin. Pityriasis alba mostly occurs on the skin of black children.
According to WebMD, vitiligo occurs when pigment cells are no longer able to produce pigment and soon blend together. There is no determined cause for vitiligo, but researchers believe that it's related to heredity and immune system problems. Pityriasis alba is a form of eczema that often appears on the face and arms although it can appear anywhere on the body. Unlike vitiligo, it's not permanent and is remedied by topical skin treatments.