Blue: Aircraft displayed as blue icons are currently being tracked via satellite. Satellites are collecting the ADS-B signals from aircraft and transmit them to the Flightradar24 network. Space-based ADS-B tracking will be used to supplement our terrestrial receiver network.
Yellow: Aircraft displayed as yellow icons are being tracked via our terrestrial receiver network or are in an estimated position.
Red: When an aircraft is displayed as red that means that you are currently following that specific aircraft on the map or that the aircraft's transponder is squawking an emergency code. These codes are 7500 (Hijack), 7600 (Communication Equipment Failure), 7700 (General Emergency)