When we look at a head of hair, one of the key parameters of its health is shine. Curiously, the shine of hair is somewhat of an optical illusion.

Darker hair shows a greater difference in the contrast between light areas and dark areas. Thus, black or dark brown hair will generally appear more lustrous than lighter brown or blonde hair.

Notice that in many advertisements for hair products, a brunette or black-haired person is usually shown. This is because a big part of our perception of luster comes from contrast (the difference between the light areas and the dark areas). Since this contrast is more noticeable on darker hair, dark hair usually looks shinier than light-colored hair.