The "Green/Blue Screen", or Chroma Key effect is an effect that can be added to a track or event by clicking on the FX button in the respective area. Once in the plug-in chooser, add the Chroma Keyer plug-in to the chain. There is a Green Screen preset that can be chosen to do true Green Screening, otherwise, you can adjust the color to match the screen that you chose to use. Once you have chosen the correct color, all areas that fall within that color threshold will become transparent and the underlying video tracks will show through.