The physical strength attribute of haki works as an "invisible armor" called Busoshoku Haki. The raise in density & thus, black coloring of a body part is in this haki category called Busoshoku: Koka.
According to Oda, haki is invisible but for viewers Oda did colour black so people can see it. Haki has slowly become one of the most-praised aspects of post-time-skip One Piece.
Conqueror's Haki is the rarest among the three haki types and only one in a million in the One Piece world is born with this superhuman power. This haki enables the user to project their energy to overwhelm the willpower of others around them. Who had the first Haki?
The black version of Haki seen after the timeskip is called "Koka" (lit. Hardening) which is implied to be a stronger version of the regular 'invisible' Haki. We see Luffy use the invisible Haki a few times, along with other characters after the timeskip so it's still around.
In the anime, some Haki users are shown surrounded by different color auras, (such as red for Luffy, purple for Doflamingo or pink for Big Mom) even if they are not using Haki at that moment.
White Haki spirit color is the rarest color and cannot be obtained with robux.
In the anime, the user of Haoshoku Haki has a dark purple aura around him.
Akinwale Oluwafolajimi Oluwatope Arobieke (born 15 July 1961) is a British convicted criminal. Known locally in North West England as Purple Aki, he is a 6 ft 5 in (1.96 m) tall bodybuilder who weighs 22 st (310 lb; 140 kg).
Appearance. Zoro is a tall, muscular young man with lightly tanned skin and black eyes. He always carries his three swords along with him, bundled up with a green haramaki over his right hip, allowing him to easily draw them with his left hand.
This type of Haki cannot be attained through training and only one in several million people are born with this ability.
Dragon is almost certainly going to be a user of Conqueror's Haki as well just like his son. In Marineford, Luffy's Conqueror's Haki was attributed to Monkey D. Dragon, meaning he is already implied to be its user despite not having used it so far. In the future, fans will certainly see him make use of this ability.
Roronoa Zoro learned how to imbue his swords with Armament Haki during the two-year timeskip when he trained under Mihawk. Thanks to his guidance, Zoro has become incredibly good at this skill.
In the anime, when Roger uses advanced Busoshoku Haki, it is shown as a red-colored flowing aura.
Using Armament Haki does turn Zoro's blades black, but unlike Yoru, they do not stay this way.
So this purple tint to his haki could've been his spirit's nature, the same to Luffy where he got his own shade. This concept may rely on the anime but just remember that the anime does have connection with Oda. So it is very likely he told them to make it in that way.
Queen is capable of using Busoshoku Haki to harden his arm in order to defend against attacks. he was first Seen using it to harden his arm to block one of Sanji's attacks.
Roger used advanced forms of conqueror's haki as he imbued his sword with conqueror's haki and clashed with Whitebeard. He also used advanced armament haki for more powerful sword attacks. Additionally, Roger possessed the observation haki, thus, making him the strongest haki user of all time in One Piece.
After the two-year timeskip, Sanji has managed to learn how to use Haki, and currently, he is a user of both the Color of Observation and the Color of Armament. So far, Sanji has not shown the ability to use the Color of the Supreme King, also called Conqueror's Haki, however, he could very well have this power.
The Haki Haki no Mi (覇気覇気) is a Paramecia-type Devil Fruit in which the user is able to see all three types of Haki, as symbolized by a different color. Haki (覇気) meaning “Ambition”, in both Viz Manga, and the FUNimation dub called the Color-Color Fruit and it was eaten by Gozu.
The 3 legendary colours (Winter Sky, Pure Red, and Snow White) are required to summon the raid boss Rip indra (Raid Boss).
Garp and Tsuru are the only two vice admirals to have special shoulder-pads; Garp is golden and blue (black and red in the anime) while Tsuru is purple with white dot whereas the standard color is blue and red.
Nami's Conqueror's Haki
Once awakened to it, Haki can be manifested as the Color of Observation and the Color of Armament. The Color of Observation grants a certain sixth sense to the user and allows them to sense things like incoming danger, power levels, intent, and emotions, among many other things.