Apple Shake 4 - Manual (Page 690)

690 Chapter 24 Keying
Using Color Replace—Method Two
A better technique is to use ColorReplace to mask a color correction. Replace
ColorReplace2 with a Monochrome node, then pipe Primatte directly into it. Next, attach
the output of ColorReplace1 as the Monochrome1 mask, and turn on the affectAlpha
parameter in ColorReplace. This process turns off all saturation in the blue areas and
turns those areas gray. This is good since the eye can detect luminance levels better
than saturation levels. The skirt and the shirt now look good.
As an alternative to Monochrome, you can use the AdjustHSV or Saturation node.
SpillSuppress
The SpillSuppress node mathematically evaluates each pixel and compares the blue and
green strength. If the blue is significantly stronger than the green, the blue is
suppressed. Because of the luminance difference between blue and green, the
SpillSuppress node tends to work better on blue spill than green spill. It also tends to
push your images to a yellow color.