Lambert’s second son Constant Lambert (1905–1951) became an important English composer and conductor, and a pioneering exponent of jazz techniques. He won a scholarship to the Royal College of Music in 1922, and he was the first English composer to be commissioned to write a ballet score for Diaghilev’s Ballets Russes. In 1929 his choral work in the jazz idiom Rio Grande was performed by the Hallé Orchestra to great acclaim. He was the musical director of the Vic-Wells ballet from its creation in 1931 until 1947, and then musical adviser for the Sadler’s Wells ballet from 1948. His ballet music includes Horoscope (1938) and Tiresias (1951). He was also a witty critic of contemporary music, most notably publishing his views in Music ho! (1934). Amusing and iconoclastic, he was the prototype for the character Hugh Moreland in Anthony Powell’s A dance to the music of time .

Lambert created a strong portrait of Constant, using energetic brushstrokes. He carefully modelled the head to capture the fresh, youthful charm of his subject. He presented his son’s slight figure somewhat vulnerably perched on a stool, with his eyes looking into the distance distractedly. He contrasted the bright blue of the backdrop against the golden yellow of Constant’s stockings.

Constant was a model for many of Lambert’s group compositions, but this was Lambert’s first portrait of him, painted when Constant was eleven. At this time he was a scholarship student at Christ’s Hospital school at Horsham, Sussex, where students were referred to as Blue-coats on account of their distinctive dress. Lambert would appear to have painted it soon after Constant started at the school (in September 1915) and before he became ill with osteomyelitis.