One of the more common questions about the anime and manga series One Piece pertains to the color of Monkey D. Luffy's Haki. The following article will go over exactly why Haki is red-colored for Luffy in this particular show and more of what that really stands for.
Visualization of Conqueror's Haki
Luffy's red Haki is usually associated with his Conqueror's Haki or the Haki of the Supreme King. In anime, it is visualized by his eyes glowing red and having red rings of energy coming out of Luffy's body. It makes it easy to tell Luffy's Conqueror's Haki apart from the rest of the users and/or which kind.
Interactions with Devil's Fruits
One theory suggests Luffy's Haki is red from an interaction caused by the powers of the Devil Fruit. An example would be when Luffy activates Gear Fourth— the red aura that contains his arms is thought of as an influence from his rubber properties on how his Haki is colored. This might be how Luffy's Haki is developed: his Devil Fruit power in combination or contrast to enhance his Haki.
Influence From Gear Second
Some fans hope that the red may be associated with Luffy's Gear Second technique. With the increased blood flow and, more importantly, the heat that this form allows for, perhaps it would affect the visual representation of his Haki. This theory seems to correlate the physical growth in Luffy's body with color.
Artistic Visual Aesthetic
On a meta level, the red color of Luffy's Haki can be written off as a stylistic decision by the anime creators to give the character a little more edge. It now serves as a much greater contrast for his on-screen character design, especially that of his red shirt, repeatedly placing his Haki invocation into a much more visually striking appearance in order to dazzle and underpin.
Generic Haki Colorations
The normal visual aspect of Armament Haki (Busoshoku Haki) is black. In Luffy's case, his Haki is red, and this might say something about his own type of Haki or how it mixes with his Devil Fruit powers.
As a Symbol and Part of Character Theme
The red color may also be symbolic to the use of Luffy's Haki. In most cases, red is known to show the quality of passion, determination, and strength, which plays very critical roles in Luffy's character. These qualities could easily be reinforced by the red color used to illustrate Haki.
Rendering of Haki Over Time
Over time, Haki became more detailed as a series progressed, often having this red coloration developed around Luffy's Haki within its advancement form, tended to show in combination with Gear techniques, this could be simply part of this development in the way that Haki is drawn throughout the anime.
Fan Theories and Speculations
The unique color of Luffy's Haki has led to all sorts of fan theories. Some even go further to speculate on varying kinds or gradation levels of Haki, where the color might diverge in terms of power levels or special attribute assignments. Of course, they are just suppositions established among the community, so far from being confirmed inside the official canon of the series itself.
The One Piece anime serial has put particular consistency in using red for Luffy's color of Conqueror's Haki, which communicates to the audience a very strong signal. This sets up in the audience the peculiar Haki signature of Luffy throughout the series.
Briefly, the color of Luffy's Haki in red is always distinguished from other characters' Haki throughout the serial. While the series never quite gives a reason for such coloring, it's likely a mixture of stylistic choice, symbolic representation, and the unique interaction of Luffy's Devil Fruit abilities with Haki. The vaunted character design of Luffy now carries itself with an extra layer of depth because of visual flashes in One Piece.