White tattoo pigment is pure titanium dioxide in a suspension of water and glycerin. It tends to have(on a microscopic scale) a larger particle size than that of other colors' pigments. This lends to a little more difficulty in application for tattoos. It's also the reason why white ink separates in the bottle before any of the other colors. Tattoo ink is deposited in the layer of skin that's below the layer that contains your natural pigment. As such, the more pigment a person has (the darker the skin) the harder it is to see the ink. Therefore, as the complexion gets darker, higher contrast colors (black, red, blue, green) show up better than lighter colors (white, yellow, gray). This is why lighter, more gradual colors show up more clearly on light (fair) skin.