Apple Shake 4 Tutorials - Manual (Page 197)

Chapter 6 Using Primatte 197
5 Connect the Blur node to the replaceImage input (the last input) on the Primatte1 node.
Since the black chair backing has lost its definition, the shirt appears less transparent.
The alpha channel remains solid.
An alternative to spill suppression is to set the replaceMode parameter to “use color,”
which fills the spill areas of the image with a solid color specified in the Replace Color
parameter.
To control spill with the “use color” replaceMode setting:
1 Make sure the composite is loaded in the Viewer.
2 In the Primatte1 parameters, toggle replaceMode to “use color.”
3 Click the Replace Color control, then scrub the color of the wall.