With a new album out, Sting talks to Craig McLean about ageing, rock ’n’ roll parenting - and being a milkman's son turned Picasso-owning millionaire. "I get confused when people say 'purple’,” murmurs a man dressed in fashionably battered blacks and greys (knackered trainers, distressed jeans, holey jumper). “Anything that’s slightly in the gaps of the spectrum, I can’t really figure it out...”