Umber is a conundrum, a rusty riddle of a colour. A cross between the deep redness of blood and the blasé blandness of mud, umber is a havering hue with a haunting humidity all its own.

Umber is the inexhaustible bog from which the 17th-Century Dutch master dredged the faces and stares of those who sat for his soulful portraits. Rembrandt created almost 100 self-portraits, ...