Right click on image and choose "script-fu" - "MH" - "cutout"(mode:0, background:black).
Duplicate image layer and select under layer. Right click and choose "Image"("Layer" on GIMP2.2, "Color" on GIMP2.4)" - "Color" - "Hue-Saturation". Set
Saturation number 45.
Choose top layer and right click, choose "Filter" - "Edge-Detect" - "laplace".
Right click on same layer, choose "Image"("Layer" on GIMP2.2, "Color" on GIMP2.4)" - "Color" - "Threshold".
Set left number 127 to 255.