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 of the Ishihara colour perception test. The beginning of the book contains a quote from philosopher Alfred North Whitehead on the subject of colour: In a 2009 interview, the author mentioned two further books in the series. The titles "Painting by Numbers" and "The Gordini Protocols" have been suggested; however, neither is scheduled for publication as of June 2017. In a webchat, Fforde said a prequel to the series: "7 Things to Do Before You Die in Talgarth", may be published in 2019. Shades of Grey Shades of Grey: The Road to High Saffron (2012, simply titled Shades of Grey