Answer:
Most aircraft that leave trails aren't leaving smoke. It's condensed water vapour; essentially, clouds. These are called contrails. This can be caused by the water in the engine exhaust condensing, or by the disruption of the air by the passage of the aircraft triggering the condensation of water that was already present. Some aerobatic aircraft have smoke generators which leave much thicker and more visible trails which can be turned on and off and don't depend on particular atmospheric conditions. White is most common but other colours are possible.