Automatic Versus Manual Keying
At its default settings, the Keyer filter attempts to work as automatically as possible. When
you apply the Keyer filter to an object, Motion analyzes the visible frame in the Canvas
to detect a dominant color such as a green or blue background. The dominant color
becomes the initial sample that generates the tolerance, or core transparency, of the
underlying key; this key is rendered as transparent in your composite.
However, you can override the automatic initial color sampling to key any color. You do
this by setting the Strength parameter to 0, then using the Sample Color tool to draw a
selection box over the region containing the color you want to key.