This is a feature request to make possible the coloring workflow used for anime style drawing.

The idea is:
-In this kind of drawing, besides the black art lines, we usually use other colored lines to mark the areas for shadows and highlight (for example, an other color can be used for objects with different color areas without black lines between them..)
-In this case, it is very important to be able to include some of these special "area-mark colors" in the fill tool result.
-Also, for this kind of workflow, we usually use perfectly-sharp-pixels lines, so it should be easy to include exactly those color-line-marks in the fill area.

Implementation proposal:
-Add a new option in the fill tool to "include selected colors", along with 3 or more color patches with a checkbox on each. By default, they should be pure blue, pure red and pure green (and others empty by default).
-When "include selected colors" is activated, all the colors from selected patches are included in the fill tool result.