Using Chroma Key for a green screen effect
Chroma Key is used to remove a specific color from an overlay clip. For example, if you capture a subject against a “green screen” background or cover part of a subject with green, Chroma Key can be used to remove the green. You can then place the subject on another background to change the surroundings or create a special effect such as the appearance of a hovering object.
You can use Chroma Key to replace the background behind subjects captured against a green screen.
To use Chroma Key to make selected colors transparent1 In the Timeline, select an overlay clip.
2 In the Options panel, click the Chroma Key tab.
3 Enable the Chrome Key check box.
4 Use the eye dropper tool 
to pick a color that you want to make transparent in the Preview Window.
to pick a color that you want to make transparent in the Preview Window.5 Adjust the color similarity value (to the right of the eye dropper tool). The transparency effect appears in the Preview Window.
6 If you want to crop the edges of the overlay, you can type a value in the Width and Height boxes. The value is a percentage of the full dimension.
If you want to fine-tune how the overlay blends with the background, adjust the settings on the Blend tab. For more information, see Blending overlay clips with the background.