The Color to Alpha filter replaces selected color information with alpha . The filter will attempt to preserve antialiasing information by utilizing a partially intelligent routine that replaces weak color information with weak alpha information. In this way, areas that contain an element of the selected color will maintain a blended appearance with their surrounding pixels.
Color to Alpha Settings
The From Color color well is where color choice can be made for the filter. Clicking this well will provide a color selection dialog and any color can be selected from here to be used for the transform.
If selection of an exact color is required, use the Color Picker to select the correct color then drag and drop the color from the color picker to the color well in the Color to Alpha dialog.
Alternatively, clicking the color well located in the center of the Color to Alpha dialog with Button2 will display a menu that allows the selection of either the current Foreground or Background colors.