1) Open a polygon dataset
2) Find the tool from the menu and run.
3) A new layer which is colored with five colors will be created. Initial, half-arbitrary coloring is applied automatically but the color index is also saved as an "autoclr" attribute which makes it possible to adjust colors later with the standard classification tools.
How to install
Map coloring plugin can be downloaded from
https://sourceforge.net/projects/ojmapcoloring/files/. Installation is done by unzipping the plugin into /lib/ext folder of OpenJUMP. Restart is needed if OpenJUMP was running.
Map coloring plugin is currently at version 0.40 and it may have issues if the polygon layer contains thousands of polygons. Using 64-bit jre helps a lot with memory issues.