3. Lay on the Overlay
Click the Choose Overlay Movie button, navigate to your overlay clip, and click Open to import it. Now click in the colored box labeled Color To Replace; in the Color Picker that pops up, click the magnifying glass icon and use it to pick a color to remove from the Overlay Frame. Use the sliders to adjust the color tolerance (Closeness), fringing (Choke), and the cutout’s edge smoothness (Smoothing). Use the Overlay Scale slider to adjust the scale of your overlay clip, and click and drag in the Overlay Frame preview to reposition the overlay clip. Click OK when you’re satisfied with the Composite preview.
Slick’s simple tools do an excellent color key-out job.
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4. Lock It In
Back in iMovie’s interface, set the Effect In slider at the far left (00:00) to start the effect at the beginning of the clip, or just set it wherever in time you want the effect to begin. Set the Effect Out slider the same way. For example, 00:00 stops the effect at the end of the clip, 01:30 stops the effect one minute, 30 seconds from the end of the clip. Click Apply to render the effect.
Peg the Effect In and Effect Out sliders to the start and end of the clip, respectively, to give your whole clip the green-screen treatment.
