This script quickly inverts the colors on all layers in an image. You can download the script here-> FP's Color Invert All Layers Script
The preview image shows the left part of the animation with normal colors and the right side shows the colors inverted.
Download the zip file, open the zip file by double-clicking on it and copy/paste the script to your GIMP scripts folder (ie...C:\Program Files\GIMP-2.0\share\gimp\2.0\scripts). Refresh your scripts and you're ready to go. The script can be located under the Colors > Invert All Layers
Let me know if you have any trouble or need additional information.