In Once Upon a Time in Sokoto at Nottingham Playhouse, tense beaten rhythms intertwine with the voices of joyous ululation as relationships are continually tested in Northern Nigeria at the turn of the 20th century. Zinara and Nafisa juxtapose their tales about the slaver and the enslaved, husband and wives, the Boko language and cowrie shells - all of which are underpinned by the power and riches of the colour indigo.