Have you ever watched a movie and wondered, how'd they do that? From light saber battles to the sinking of the Titanic, some special effects seem almost magical. Most film makers still use a relatively simple process called matte layering, which involves using a blue or green screen to create images that can be layered to create a composite image.
Let's say you're filming a tornado disaster movie and you need a scene where the hero--let's call him Lance--is holding onto a signpost for dear life as a tornado threatens to carry him away. Since there's no way Lance would agree to brave a real tornado, you'll use some special effects trickery.
First you'll need a background plate: a shot of a tornado swirling through the sky. Then you'll need an image of Lance holding on to the signpost to insert against the tornado backdrop. That's where the colored screen comes in. In the studio, you'll film Lance hanging onto the signpost against a bright green backdrop.
When run through a special filter, the green parts of the film become transparent. You then have footage of the struggling Lance against a clear background. Lay that footage over the tornado background image and voila: suddenly Lance struggles to hold on as the tornado looms in the background. Although the two scenes were filmed in different locations, the end result looks amazingly real.