What is difference between silver, gold and legendary groups?

In the Clash of Provinces event you will start off in silver and after each season completes the top 1, 2 or more provinces will be promoted up to Gold. After future events from gold you can be promoted up to legendary or demoted back to silver if you do not do well.

The main differences are more established and stronger provinces in the higher brackets that will be harder to beat but with improved rewards available for each season.