Privately run buses are blue-green with yellow non-electronic destination signs while IETT-run buses come in many flavors including old red-blue ones, newer green ones and red double-deckers. The Akbil Transit Pass is valid universally while tickets that can be obtained in kiosks near bus stops for 1.4 YTL are valid only on IETT buses and cash payment only on private buses, although if you get on an IETT bus the driver will normally accept cash (normally 1.5 YTL but this is dependent entirely upon what the driver wishes to charge) and hand you his Akbil for you to use.
Ils se prennent comme un métro à Paris: indispensable.