Use Google's Squoosh.app: https://squoosh.app https://squoosh.app When editing an image you can choose the 'Reduce palette' option to bring up a way to reduce

Use Google's Squoosh.app: https://squoosh.app https://squoosh.app When editing an image you can choose the 'Reduce palette' option to bring up a way to reduce the number of images used, and the palette reduction panel will let you optionally apply dithering or not. The dithering algorithm is not selectable but it's worth trying to see if you like it.