Why are some aircraft shown as yellow or red on the map?

Yellow aircraft icons: 

Aircraft shown in yellow are being tracked in real time via our global receiver network. In some cases, their positions may be estimated. 

Red aircraft icons indicate one of two things:

• You have selected the aircraft and are currently following it on the map.
• The aircraft’s transponder is broadcasting an emergency squawk code:
  • 7600 – Radio failure (lost communication)
  • 7700 – General emergency