Two reasons to rejoice at the Donmar: a really good play about art (Art is about friendship, not art), something of a rarity, by an unknown writer, John Logan, and the overdue return to the stage of Alfred Molina as the gloomy dauber Mark Rothko.

Molina’s already in situ when the audience enters, contemplating his big red squares and black blodges in designer Christopher Oram’s brilliant evocation of the painter’s studio, full of frames and canvases, in the New York Bowery, in 1958. Like Timberlake Wertenbaker’s more flaccid piece about Edgar Degas at the Arcola, there’s the device of an apprentice coming to call, in this instance Eddie Redmayne’s eager, callow Ken, whom Rothko wastes no time sorting out on his lack of experience and narrowness of reading.

Much of the play, thanks to the intensity of the acting in Michael Grandage’s beautifully weighted production, is a fiery debate about the purpose and impetus of art, avoiding banalities as it’s couched in a discussion of Picasso being “stamped on” by the abstract Expressionists, who in turn are ceding ground to the Pop school of Andy Warhol.

But the dramatic hinge, something lacking in the Degas play - though, interestingly, both Degas and Rothko rail against painting in the open air (unlike David Hockney in Bridlington) - is well oiled around Rothko’s commission to hang his murals in the Four Seasons restaurant at the new Seagram Building on Park Avenue, and his doubts about the clientele.

Ken is hired on a nine-to-five basis but is soon standing up to Rothko, mixing paints and, in one wonderful scene - physical theatre in excelsis - transforming a blank canvas to a mulberry abstract as they grapple graphically with brushes; the soft background of Mozart and Schubert on the record player erupts in a joyful baroque chorale.

Molina, physically momentous, shaven-headed and deeply concentrated, challenges Redmayne to match him with his own diatribes, enthusiasm and sad account of parents in Iowa who were murdered; this episode of snow, blood and urine is almost a composition in itself.

Selling a painting, says Molina’s Rothko, is like sending a blind child into a roomful of razor blades. It’s a chilling metaphor in a marvellous piece of theatre, too, about the artist’s relationship with his patron, a theme perfectly suited to the smartly sponsored Donmar and its audience.