Black Skin. This is not a joke! Those of African origin, certain Orientals, or pacific Islanders might test positive for marijuana. Dr. James Woodford, a toxicologist associated with Emory University labs hypothesized the pigment melanin which protects the skin from the sun, approximates the molecular structure of the THC metabolite to cross react on the marijuana urine test. Dark skinned Caucasians such as those from the subcontinent of India could also read positive on marijuana tests. The body eliminates some melanin in a dark person's urine sample.