Changing color of an MKOverlay that has already been added to a mapview

I have some MKOverlays(actually they are MKPolygons) that are loaded as soon as the map shows up. I would like to change their color dynamically. The only way I can see of doing this, is to remove the overlay then add it back with the new color. Is there a better way to do this on the existing overlay?

I am brand new at objective-c/xcode/ios ... so please be gentle :)