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
Shades of Grey the town close to the lower of a series of five dams suggests it is Rhayader, at the foot of the Elan Valley. Nearby Rusty Hill was once Builth Wells. The town of Vermillion used to be Hereford. The town of High Saffron is on the coast beyond the dams, which suggests Aberystwyth. The colour values as described in the book supposedly come from the Munsell color system as described by Albert Henry Munsell, but are derived from the HSV color model. The "Ishihara", a test used to determine one's colour vision, is a reference to Shinobu Ishihara, the inventor