Typical face washes do not brighten the skin. A face wash that does claim to do it usually does so by having a mild chemical exfoliant in the product. This works by breaking the "glue" that makes the old top layer of the skin stick to the younger lower layers. When this glue is broken the top layers are removed and the younger more vibrant layer is exposed, thus "brightening" the skin. Brightening means giving it a more youthful appearance, it doesn't necessarily mean that it will change the color of the skin drastically.