In many languages, the colors described in English as "blue" and "green" are colexified, i.e., expressed using a single umbrella term. To render this ambiguous notion in English, linguists use the blend word grue, from green and blue, a term coined by the philosopher Nelson Goodman – with a rather different meaning – in his 1955 Fact, Fiction, and Forecast to illustrate his "new riddle of induction".
The exact definition of "blue" and "green" may be complicated by the speakers not primarily distinguishing the hue, but using terms that describe other color components such as saturation and luminosity, or other properties of the object being described. For example, "blue" and "green" might be distinguished, but a single term might be used for both if the color is dark. Furthermore, green might be associated with yellow, and blue with either black or gray.
According to Brent Berlin and Paul Kay's 1969 study Basic Color Terms: Their Universality and Evolution, distinct terms for brown, purple, pink, orange, and gray will not emerge in a language until the language has made a distinction between green and blue. In their account of the development of color terms the first terms to emerge are those for white/black (or light/dark), red and green/yellow.

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