The tileset image should use a palette that has room for the colors used by the sprites. You can read about how to do this at Disguise's Tileset Resource, or by opening the "tsProperties.htm" and "tsPal.htm" and "tsManual.htm" pages in your JCSHelp folder. If you're having trouble finding graphics editor software that can use palettes, you can always check the "Remap tile set palette" checkbox for a serviceable, if not perfect, solution.