There are three Colours of Haki: Armament, Observation and Conqueror. But what is Haki? You can literally translate it as Ambition or Willpower. I think Haki has a strong relation to character’s ambitions and dreams. Depending on those two, you can ‘personalize’ your Haki. Personalization of Colours would be known as Shades.
Rayleigh said that some people specialize in one or two Colours of Haki, and I think that would mean not just using the Colour but also mastering the Shade.
The stronger your will and dreams are, the stronger your Haki and Shades will be.
I will list possible Shades that were shown in the entire series:
– Enhancement – the invisible Haki that’s used, for example, by Kuja pirates. It can be imbued inside weapons or your body. It was shown when Luffy punched Pacifista, Boo tried to attack Funk Brothers with his haki imbued axe and Zoro cutting Monet’s cheek.
– Hardening – advanced form of Enhancement, it’s basically a black ‘armor’ surrounding your body or weapon. It’s stronger than Enhancement and can be seen by people who posses Observation Haki. It’s been used multiple times in New World. Anybody could achieve this and Enhancement, but you can personalize Hardening (for example, Luffy having Haki tribal tattoo on his body in Gear Fourth, or swirly Haki outside Zoro’s swords)
– Elemental – the most advanced Armament Shade known so far, it allows you to create an element (which can be personalized) and use it in combat. Examples: Luffy’s Red Hawk, Sanji’s Diable Jambe post timeskip, Kin’emon cutting fire, Zoro igniting Ryuma, Vista’s flowers, McGuy’s electric swords
– Presence – being able to sense other’s presence or auras. Also known as Mantra.
– Clairvoyance – being able to tell what is going to happen. For example, Luffy unconsciously used it in Marineford when fighting Mihawk. Maybe that’s how Madame Shyarly can tell the future?
– Voice – only three known people that can access it, that is: Luffy, Roger and… Coby. Luffy and Roger could hear Sea Kings, Roger could hear Poneglyph’s voice, Coby can’t do that stuff, he can hear human’s voices very well.
– Intimidation – knocking out unconscious opponents [much] weaker than you and paralyzing them with fear
– Charisma – ability to affect matter and energy, for example, Shanks destroying WB’s ship by passing by, Ace, Roger and Luffy vs Doffy CoC clash extinguishing fire, Dragon controlling winds in Lougetown (this could be a DF but it could be haki too)
People with strong will and high ambitions have unlocked stronger Shades (e.g. Luffy and Roger dreaming of finding One Piece and being very confident about it have unlocked Voice) and their Haki would be stronger as well. It’s possible that there might be more Shades as well.
*Theory by Athashet