They drop all four charms, expect a large quantity of golds than any other but between crimson/blue/green it is pretty even. I actually tried to collect every single charm they dropped, and I had more blues than gold, crimson or green :D P.S. it's not just these zombies that drop blue charms that often, the skeletons and zombies in chaos tunnels are great blue charm droppers too and it's a perfect training spot as well, because it's multi and you can use a combat familiar for example I use addy minotaur for extra def xp because I'm training def. How ever, theres no altars out there.. They don't hit that much but if you wanna use something like piety, you gotto bring prayer pots.. Anyways, blue charms ftw :D