To make the video monochrome except one color, apply the Secondary Color Corrector. Use the Select Effect Range eyedropper to select the color you want to keep. Then, choose Invert Mask, then lower the saturation.
You can apply any effect and leave an unaffected rectangle by putting a copy of the video above itself, apply the effect to the top video and also apply the cookie cutter to make a hole that the unaltered video will show through.
Gary Kleiner
