"....he saw them quite clearly in their little sitting room, the tall, black-haired man in his glasses, making puffs of coloured smoke erupt from his wand for the amusement of the small black-haired boy in his blue pyjamas."
-- Lord Voldemort in Godric's Hollow (DH17)
A charm to make coloured smoke rings emerge from the tip of one’s wand.
References from the canon
Harry Potter is linked to Lord Voldemort's memories while he remembers the previous time he had gone to James and Lily Potter's home in order to kill baby Harry. One of the memories is James entertaining Harry with coloured smoke rings (DH17).
A similar effect may have been used to enchant the Dissimulator, a "musical" instrument introduced at the 1974 Quidditch World Cup, which featured loud cries of fan support and gave off puffs of smoke in the national team colours (Pm).
From the Web
About the Harry Potter and the Cursed Child play: http://www.harrypottertheplay.com