Pigmented birthmarks are usually brown patches on the skin caused by clusters of pigment cells. Examples of pigmented birthmarks include café-au-lait spots, Mongolian spots and congenital melanocytic naevi. Pigmented birthmarks are different to vascular birthmarks, which are usually red, pink or purple and caused by abnormal blood vessels. The cause of pigmented birthmarks is not known. However, the amount and location of melanin (a substance that determines skin colour) determines the colour of pigmented birthmarks.