Maybe it will be overkill for flags, but you can try with dithering algorithms, where you can select which color you want to keep: Https://ditherit.com/ Https://github.com/noopkat/floyd-steinberg. Source: over 1 year ago

ImageMagick can do anything, but it's a commandline program. There may be some websites that dither images for you - https://ditherit.com/ is one. (I also wrote a program years ago that is excellent at color reduction and dithering, but it's not currently usable.). Source: almost 2 years ago

It’s called dithering. Instead of using many different colors you just choose a few and make the best of it. With using six colors (red, green, blue, yellow, white, black) any color can be matched. You can try it out with this tool: https://ditherit.com/. Source: almost 2 years ago

I used https://pixelartselfie.com/en. I later found https://ditherit.com/ which offers more features so I’ll probably do more works. Source: almost 2 years ago

Check this website: https://ditherit.com/. Source: about 2 years ago

I've tried a few, the best one was pingo (https://css-ig.net/pingo). Pingo -s9 gives better results than oxipng with Zopfli, while being usually two order of magnitudes faster. It's also faster than "regular" oxipng while being better. I can usually shave of 15%/20% of the size of png files I encounter. One thing I didn't check is that you might pay that in decoding time, I've never seen anybody talking about that... - Source: Hacker News / about 2 years ago

Pinga, GUI for Pingo (https://css-ig.net/pingo) for lossless photo compression while keeping the metadata. Source: almost 3 years ago