Similar to the Adjust filters, the color filter are image wide color operations.
Color to Alpha
This filter allows you to make one single color transparent(alpha). By default when you run this filter white is selected, you can choose a color that you want to make transparent from the color selector
The Threshold indicates how much other colors will be considered mixture of the removed color and non-removed colors. For example, with threshold set to 255, and the removed color set to white, a 50% grey will be considered a mixture of black+white, and thus transformed in a 50% transparent black.
This filter is really useful in separating line-art from the white background.
This filter converts the colors of the image to colors from the reference image. This is a quick way to change a color combination of an artwork to an already saved image or a reference image.
This filter checks for all the channels of a each single color and set all but the highest value to 0.
This is reverse to Maximise channel, it checks all the channels of a each single color and sets all but the lowest to 0.