Marge Simpson, Mrs Slocombe, Lady Gaga in her blue period: these are not typically the muses I’d bring up as inspiration at the hairdresser. Yet, here I am, staring into the mirror at the upmarket Errol Douglas salon in Belgravia, London, fascinated as my colourist Rafael strokes bright blue product onto my hair. How blue? As blue as a Caribbean sea, a Smartie, lapis lazuli, a lobster pre-boiling water.