Any drawing or animation program needs an alpha lock feature. It's annoying to draw something and realize you need it in a different color, as the fill bucket doesn't work for that.
Alpha Lock is just locking the opacity of each pixel of a layer so it can be drawn over to change the color of your lines. For example, if you drew something in blue that you need to change to red, just activate Alpha Lock and run over the entire image with a large red brush.
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