Gold and crimson charms frequently drop from Zamorak warriors.
As for green and blue charms, it's pure luck if i'm honest. Just kill some not too hard high level monsters and start praying. If you have a slayer task, or want some range xp, or just want to try your luck, try Black dragons. They drop 3 charms at a time, sometimes blue, which is a very good rate due to the rarity of blue charms.
Also, use any all in one fighter and set it up accordingly. My reccomendation for a free bot would be on from KBot, and my reccomendation for a paid bot would be PerfectFighting from rsbotting.com