The number of melanocytes, their spacing, and the extent of their response to sunlight are genetically determined. We had a number of class comments about melanocytes. Here are some of them. The drug is doxycycleine. During pregnancy, hormones change and a person may look like they have a sun tan even if they don't because the hormones have an effect on melanocytes in certain places on the body.
Albino people lack the enzyme needed to convert tyrosine to melanin.
The enzyme is called tyrosinase. Freckles are caused by clustering of melanocytes. Tattoos or anything that stays and doesn't get renewed along with the topmost layer of skin must be in the dermis. So a tattoo involves putting pigment into the dermis. Moles are partly or all in the dermis.