Working with an alpha channel mask in your image you can exclude the color separation in the masked area.
1. The white of the alpha channel is detected as a mask.
2. In the plug-in, you can exclude the color separation from the option 'Mask' where you can select the alpha channel mask.
3. Once the mask is applied, the separated color will be removed from the separation.
4. Using the inverted option, the mask selection will be inverted and applied to the selected color.