The Color Key effect keys out all image pixels that are similar to a specified key color. This effect modifies only the alpha channel of a layer.
When you key out a color value in a clip, that color or range of colors becomes transparent for the entire clip. Control the range of transparent colors by adjusting the tolerance level. You can also feather the edges of the transparent area to create a smooth transition between the transparent and opaque areas.
Click the Key Color swatch to open the Adobe Color Picker, select a color, and then click OK.
Click the Eyedropper icon, and then click a color on the computer screen.