Lavender or Self-blue refers to a plumage pattern in chickens characterized by a uniform, pale bluish grey color across all feathers. The distinctive color is caused by the action of a recessive gene, commonly designated as "lav", which reduces the expression of eumelanin and phaeomelanin so that black coded areas of the plumage appear pale grey instead, and red coded areas appear a pale buff. (Crawford 1990, p. 139-140)
The uniform grey color described as Lavender is based genetically on a black colored bird; when the lav gene is expressed in a bird with red and black Mille Fleur plumage, the resulting cream and pale grey color scheme is called Porcelain.
The lavender gene was first discovered in the Porcelain variety of Belgian Bearded d'Uccle bantams in 1972, and verified in 1980 (Crawford 1990, p. 139-140). Porcelain colored d'Uccle bantams were around as early as early as 1909 (La Basse-Coeur du Nord 2008), though the Porcelain variety was not recognized by the APA until 1964 (belgianduccle.org). Whether from the Porcelain d'Uccle or other, unknown sources, the lavender color has been introduced to a number of new chicken breeds over the years, including the Polish and the Silkie.