Fish tree paella

How long does it take for a birthmark to appear ? and why would it change shades ? - why would skin appear mottled

my almost 4 years later, one morning, saw a boy that I had a pigment of the skin in the neck right .. It was really black for over a month, the true light. I showed it to many people who all say, say, a mole and the light must really why I never did.