Artist: Henri Matisse (1869-1954) painted a world suspended in time, at once modern and archaic. He was in his 30s when the 20th century began, entering middle age when his rival Picasso painted the revolutionary Demoiselles d'Avignon in 1907. By then Matisse was established as leader of the avant-garde group known as the fauves, or wild beasts, proselytisers for pure colour. And yet the world they saw was recognisably the same as that painted by the impressionists when Matisse was a baby, a 19th-century bourgeois sphere of elegant pleasure.
Matisse was not thrown by Picasso and Braque's more extreme modern movement, cubism - or only a little. He experimented with flattenings of space, while never abandoning his study of colour, and his art emerged as a unique, inimitable form of modern abstraction.
Matisse's paintings are often seen as carefree idylls, yet for him colour is an emotional language. He communicates difficult things through planes of grey and green, blue and black. His painting The Piano Lesson (1916) uses an idiosyncratic cubistic abstraction to convey a sense of emotional torture, as a woman on a high stool watches a boy at the piano pinned in place by planes of flattened colour. The mood of this wartime painting could not be more different from The Red Studio, a rhapsodic transfiguration of the painter's room.
Subject: Lydia Delectorskaya was close to Matisse in his later years, employed as his wife's companion and his assistant. Her book l'Apparente Facilité is an eyewitness account of his methods. Asked how Matisse's drawings seem to have been done in a single flourish, when he was a patient worker, she said she was "a pretty good eraser".
Distinguishing features: There's a rawness to this painting that's amazing in an artist in his late 70s who might have been expected to sink into self-regard. It is gloriously free, with a wonderful mixture of sophistication and directness. Lydia is a much younger person, to whose energy the old man pays tribute with a controlled romanticism; her elegance and bold, easy look are captured in equally confident lines.
The colours are those of life. Her hair is bright green, the colour of spring, renewal. Around her blazes an orange fire of fizzy energy. Her face, divided into blue and yellow, is like some heraldic image from a tarot pack. She's a totem of youth and beauty, a modern beauty, off- kilter, in motion, as Matisse offsets the classical harmony of Lydia's powerfully drawn features with the unkempt fuzz of green hair to the left, blowing in the breeze.
This is a painting at once abstract and personal, its simplicity deceptive. The particoloured harlequin character of the painting makes it something other than a description of a person. The colours have their own life and mystery; the left-hand area in particular almost turns into pure abstraction as we get engrossed in that weird, jagged black border between the thickly built-up orange and more mobile green.
Inspirations and influences: The pattern recalls a work Matisse hugely admired, Picasso's Harlequin (1915), as well as the flattened, parti-coloured paintings of Russian peasants by Kasimir Malevich.
Where is it? French Drawings and Paintings from the Hermitage at Somerset House, London WC2 (020-7845 4600), until January 20.