Dithering
To be able to use a palette with a limited number of colors, the number of colors used to form an image must be restricted. To obtain a color needed for an image that is not in the palette, you may mix available colors to obtain a desired color.
To start dithering, run the dialog by pressing "Effects -> Dithering" on the user menu.
In this dialog you can choose a palette preset. The following presets are available:
The "Adaptive" option forms a palette, the use of which would minimize image distortions.
After the palette is chosen and adjusted, the image will be adopted to it through dithering.