One of the more popular effects on television is called a "chroma key." To create this effect, one color in a shot is replaced by a second image.

This is a staple of TV weather. The weather forecaster stands in front of a big green screen. The green is replaced by a second image--the weather map.

Of course, this all happens electronically; the weather forecaster is still standing and motioning in front of a big green screen. But he sees the combined image in a small monitor to the side. Watch closely and you will see weather people glance at this off-screen TV monitor.

This is also called a "greenscreen" effect. It's worth noting this technique is always done with a green (or blue) screen--never red or orange.  That's because blue and green are distinct from most skin tones. The effect works best when there is a clear difference between skin colors and the background color. Because whenever the chroma key device sees the background color (or something similar), it lets the second shot bleed through.

Every once in a while there is a funny example of how this can go wrong--for example, the local weather forecaster who wears a green jacket. The jacket becomes invisible and the result is a floating head!