Normal skin contains cells called melanocytes that produce the brown skin-coloring pigment melanin .
There are several conditions in which melanocytes are either abnormal or abnormally distributed. Most skin conditions that cause discoloration are harmless. Sometimes mistaken for a suntan, bronzing of the skin often develops gradually starting at pressure points such as the elbows, knuckles, and knees and spreading from there. It is also seen in the creases of the soles of the feet and the palms of the hands. This list is not all inclusive, and the causes are not presented in order of likelihood.