This filter is found in
This filter combine values of the RGB channels. It works with images with or without an alpha channel. It has monochrome mode, preview.
From this menu you select the channel to mix to. Choices are Red, Green, or Blue. It is insensitive when Monochrome option is checked.
This three sliders set the contribution of red, green or blue channel to output. Can be negative.
Sets all output channels to be the same. Makes the Output Channel menu insensitive.
Maintains the luminosity at 100% regardless of the channel slider settings.