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I just have a few questions about haki.

How do pirates get haki? Rayleigh said it could be attand through training. With is fine for the marines and luffy, Zoro, and Sanji who all had teachers who could teach them but what about pirates like law who tought him. If you don't have a teacher how do you learn haki?

Why is haki able to turn your body black now. Luffy, Smoker and Vergo all had this ability and I get it has to do with the amount of haki but why didn't we see it during the whitebeard war with people like Marco, Juzo, and vista and Garp and Whitebeard? (side note: the marines said they had 10 000 of their strongest fighters at the war, but where were Vergo, and Fujitora?)

If you learn one type of haki can you automaticlly use the other kinds. Not Haoshoku Haki ( conqueror's Haki) but Busoshoku Haki and Kenbunshoku Haki if someone learns one of these types can they use both of do they need to train in both?