Didder[0] is a good tool for dithering. I use it to pre-process images for the 6-color e-ink displays.

You can go down a rabbit hole of dithering algorithms too, like Floyd Steinberg[1]

[0] https://github.com/makew0rld/didder [1] https://en.wikipedia.org/wiki/Floyd%E2%80%93Steinberg_dither...