I've had terrible ones, covered in green. All I use is an applicator on a pole or a car brush, with soapy water and it usually comes off a treat. Wash down with a hose and (if you like) a final wash with GG4 and it'll be even easier next time.
A word of warning though, if it's a victorian style with spikes on the top, be careful as it's easy to knock 'em off. Go steady. If they do fall off, they just clip back on again but it's a pain to do from a distance.
