Similar to the Adjust filters, the color filter are image wide color operations.
This filter allows you to make one single color transparent(alpha).
You can pick the color in question with the color-picker widget. 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.
With this filter, the input image will take over the color palette of the reference image.