I want people to see this. I just posted it in a different section.

[Sinsemilla Jones]

Perhaps you're thinking of melanin, the chemical which determines skin color.

Melanin, the pigment responsible for dark skin in some peoples and for tanning after exposure to sunlight in light-skinned peoples, is produced from the "natural" indole and amino acid, tryptophan. Melatonin, which is synthesized by our bodies only in the dark, blocks the synthesis of melanin.

http://www.vitaletherapeutics.org/vtlindol.htm