I don't know if it's exactly what you're looking for but I made a free tool https://app.dithermark.com https://app.dithermark.com . It only allows 18 colors max

I don't know if it's exactly what you're looking for but I made a free tool https://app.dithermark.com https://app.dithermark.com . It only allows 18 colors max, but it has a lot of options, including the ability to determine what color palette to use.