Mythmoor has the Kingdom Sails server-side mod that lets you change ship sails. Simply craft a new sail with a needle and square pieces of cloth. To remove a kingdom sail, right click and use the menu option. Sails can also be dyed.

The kingdoms are as follows:

• Abralon
• Apocalypse Order
• Black Legion
• Dragon Kingdom
• Dreadnought
• Ebonaura
• Empire of Mol-Rehan
• Freedom
• Horde of the Summoned
• Jenn-Kellon
• Kingdom of Sol
• Legion of Anubis
• Macedonia
• Mol-Rehan
• Pandemonium
• Commonwealth
• Crusaders
• The Rome Republic
• Valhalla Descendants
• Valhalla
• Wurm University

The above list corresponds to the photo below left to right for both rows, starting with the top row. Pic courtesy of Mitty.