Every kingdom has an explore button. Just click the kingdom on the map and you will see a menu like the one above. Since this event calls for killing yellow troops in Explore, you might want to start with Whitehelm since it has the most yellow troops.
You could also try Explore in Divinion Fields and Drifting Sands since they have a lot of yellow troops. Basically, you get event points for killing any yellow troop in Explore mode in any kingdom. It’s just that some kingdoms tend to have a bigger concentration of a certain color.
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