One option in Corel Photo-Paint is Image > Transform > Posterize. Posterize isn't listed in my American Heritage College Dictionary, nor could I find a definition at Merriam-Webster Online. But here's what it can do:
Corel Photo-Paint Help says that posterize allows the user to "reduce the number of tonal values in an image to remove gradations and create larger areas of flat color."
Whatever! I just say cool.