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Plum is a purple color with a brownish-gray tinge, like that shown on the right, or a reddish purple, which is a close representation of the average color of the plum fruit.
This is the color traditionally called prune because it is a representation of the average color of prunes. Prune is the French word for "plum", but in English the name "prune" is applied to dried plums. This color is a representation of the color of cooked dried plums (prunes).
The first recorded use of prune as a color name in English was in 1789.
References
This color matches the color identified as plum in the color sample in Maerz and Paul's A Dictionary of Color, displayed on page 117 Plate 47 Color Sample J9
William J. Miskella, 1928, Practical Color Simplified: A Handbook on Lacquering, Enameling, Coloring And Painting, pp
John Lemos, 1920, "Color Charts for the School Room", in School Arts, vol. 19, pp 580–584
Maerz and Paul A Dictionary of Color New York:1930 McGraw-Hill p. 202; Color Sample of Plum4: p. 117 Plate 47 Color Sample J9 Note: The color designated plum (color #8E4585) that is displayed above matches the color identified as plum in the color sample in Maerz and Paul
Maerz and Paul A Dictionary of Color New York:1930 McGraw-Hill p. 202; Color Sample of Prune: p. 117 Plate 47 Color Sample J8