I'm trying to create a political map from country polygons with GeoServer.
Is there away to assign the countries colors from a defined set of colors without having to assign colors individually?
The result would be so that each neighboring country would be different.
I am unfamiliar with GeoServer but what you are interested in is the four color theorem. There is a four color theorem for QGIS at Achieving four color theorem for QGIS.
There is an Esri approach at Applying four colors theorem in polygon map using ArcMap.
Try adding a new column to your data, populate it with number values from the four color theorem tools, and symbolize off of that column.
