Human hair color account for all possible shades of brown, blond, or red (for example, platinum blond versus dark blond/light brown), nor does it explain why hair color sometimes darkens as a person ages. Several gene pairs control the light versus dark hair color in a cumulative effect. A person's genotype for a multifactorial trait can interact with the environment to produce varying phenotypes (see quantitative trait locus). Natural hair color can be brown, blond, black, red, or white; all natural hair colors including grey, white and lightest blond, are shades of brown. The Fischer–Saller scale, named after Eugen Fischer and , is
Natural Color System lightness value "v" as the sample on the reference scale to which it has the least noticeable edge-to-edge difference. Two examples of NCS color notation—the yellow and blue shades of the Swedish flag: The NCS is represented in nineteen countries and is the reference norm for color designation in Sweden (since 1979), Norway (since 1984), Spain (since 1994) and South Africa (since 2004). It is also one of the standards used by the International Colour Authority, a leading publisher of color trend forecasts for the interior design and textile markets. In order to be able to manufacture physical representations of