https://github.com/felix-u/imgclr
This project is several months old, but with its first actual release, imgclr has an improved README, simplified command-line interface, and static binaries available for download.
imgclr supports the most popular image formats and a multitude of dithering algorithms. It can also invert image brightness while preserving hue and saturation.

Recent image colour-changing utilities, including mine, provide roughly these same features, but there must be yet more bizarre and beautiful ways to defile augment images. I'd love some ideas!