Indigo: not only one of the seven colours of the rainbow and the dye that makes your jeans look like they do but and a highly valued pigment which is naturally found in some plants and whose use can be traced back at least six thousand years to Peru. Such was the desirability of indigo that along with sugar, cotton, coffee and tobacco it became a major driver for globalised trade and the horrors of slavery. In India it was the source of so much exploitation that a lawyer called Gandhi rose to fame standing up for indigo farmers. Rajan Datar explores the rich history of the dye with Jenny Balfour-Paul, an Honorary Research Fellow at Exeter University and author of Indigo: Egyptian Mummies to Blue Jeans; Lucille Junkere, an artist and textile researcher with a particular interest in the history of indigo in Nigeria and the Caribbean; and Andrea Sella, a professor of chemistry at University College London who delights his students with all kinds of colourful experiments with indigo. Photo: Detail of adire indigo cloth from Nigeria. Credit: Education Images/Universal Images Group via Getty Images