Done with The GIMP (took a few minutes to figure out... then about another minute to actually do all the layers). Go into Greyscale Mode (under the Image menu)... Open the Layers Dialog, and then for each layer in the GIF, do a Layer - Colours - Invert. Save (Export as an Animation) and you're done.
Done with The GIMP (took a few minutes to figure out... then about another minute to actually do all the layers). Go into Greyscale Mode (under the Image menu)... Open the Layers Dialog, and then for each layer in the GIF, do a Layer - Colours - Invert. Save (Export as an Animation) and you're done.