Warning: This article contains spoilers for Demon Slayer: Kimetsu No Yaiba
Demon Slayer: Kimetsu no Yaiba shows off a slew of Nichirin katanas. All of them being the signature of their respective wielders and a point of recognition for fan-favorite characters. Even with the second season out and rapidly viewed by fans the different sword colors have not all been revealed.
Nichirin katanas change color once they are given to their user. These colors bestow different properties on the blades. This is why powerful swordsmen have patterns on their blades as well as distinct colors. Each of these blades' power enhances the techniques and strength of the holder.
Indigo-Gray
This sword is wielded by Inosuke Hashibira, voiced by Yoshitsugu Matsuoka. Inosuke's twin blades turn indigo-grey upon receiving them, and he puts the numerous gashes into the blades himself. These gashes cause more of a tearing effect in the flesh rather than the clean-cut of other Nichirin katanas.
These gashes are also theorized to enhance his self-created Beast Breathing technique further. How Inosuke grew into a nature-enhanced individual is seen through his creation and use of Beast Breathing and the wielding of his dual katanas. Inosuke's fighting style is animalistic in nature, both in his hand-to-hand and swordsmanship styles allowing him to be unpredictable in battle.
Amber
The amber Nichirin katana color comes from the sound Hashira, Tengen Uzui. This color is not seen in the standard Nichirin katana style but rather in Tengen's personalized Nichirin cleavers. Tengen created the Sound Breathing style, bringing together his advanced hearing and various practiced techniques such as Musical Score and Echolocation.
These dual Nichirin cleavers compliment Tengen's flashy and often flamboyant personality. They clearly were personalized to fit him and other equipment he uses. The cleavers are attached by a chain and are too large to be sheathed, forcing Tengen to wrap them in cloth when they are not in use.
Yellow
Yellow Nichirin katanas are associated with one of the main breathing techniques and are seen wielded by those who practice Thunder Breathing. In the story, Zenitsu Agastuma (voiced by Hiro Shimono), Kaigaku, and their master Gramps (Jigoro Kuwajima) are the ones seen using this technique, and Zenitsu and Kaigaku have the yellow katanas.
Zenitsu, having only mastered the first form in the Thunder Breathing technique creates variations and modifications to enhance his skill and swordsmanship. His blade is adorned with lightning streaks along the entirety of the blade and it is sheathed in a simple black sheath. The yellow sword enhances Zenitsu's speed abilities when paired with his breathing technique.
Green
Green Nichirin katanas are connected to Wind Breathing users, most notably the Wind Hashira, Sanemi Shinazugawa. Although Sanemi did not create his breathing technique and the weapon that he pairs with it, he is among the most powerful of the Hashira.
With his mastery of Wind Breathing, Sanemi is considered to be the most skilled user of this technique in his generation. His green Nichirin katana is still personalized to him and his skills with it adorned with a jagged pattern similar to a beast jaw and held in a sheath covered in markings matching the scars on his face.
Gray
Gyomei Himejima does not use the standard Nichirin katana like his fellow Hashira, this is not uncommon, however, instead he uses a gray swinging weapon consisting of a flail with an ax attached to the end via a longer chain. Even with this weapon not following the standard bladed weapon, it's still forged of the same material as Nichirin swords.
This weapon enhances Gyomei's ability to fight through its enhancement of his breathing technique, and the sounds of the chain can be detected via his acute sense of hearing allowing him to further grasp the space and movements around him in battle.
Blue
The first Hashira met in the story, Giyu Tomioka (who is interesting enough to get his own OVA), wields a blue Nichirin katana. This katana falls into the standard of weaponry with it being the same size as most Nichirin blades, characterized to Giyu through its color and hexagonal tsuba. Giyu is the Water Hashira, making him his generation's most skilled Water Breathing user.
His rapid mastery of the Water Breathing technique allowed him to create an entirely new form. Water Breathing Eleventh Form: Dead Calm was created by Giyu and is showcased in the first season of Demon Slayer. This allows a state of total stillness while handling incoming attacks with undetectable speed.
Red
Red is the signature color of Flame Breathing users, such as Kyojuro Rengoku (voiced by Satoshi Hino) and his father and master Shinjuro. Rengoku was the flame Hashira and his Nichirin katana was made from scarlet ore, and like the other, Hashira had the words Destroyer of Demons engraved on the side.
Rengoku started his teachings with his father but mastered the Flame Breathing technique through careful study of the instruction books left behind by other former Flame Hashira in their family. His skilled swordsmanship is shown through his defeat of the Lower Rank Two demon and holding his ground with the Upper Rank Three.
Light Purple
A light purple Nichirin sword has yet to be seen in the anime in the manga; however, this colored blade was wielded by one of the Twelve Kizuki. Kokushibo (formerly Michikatsu Tsugikuni) holds the position of Upper-Rank One but was a former Demon Slayer.
His katana has been modified with his own flesh and blood, which only enhances its strength and abilities. Unfortunately, this modification has turned the blade a blood-red color. Still, the former purple color was connected to the Moon Breathing technique, which Kokushibo enhanced with his blood demon art to turn this technique into one of the most deadly.
Black
The black Nichirin katana first appears in this story for Tanjiro Kamado voiced by Natsuki Hanae. Black Nichirin katanas are thought to be bad luck, and it is learned that this is due to users passing before too much can be learned about the blades. The properties that the black katanas hold is almost entirely unknown due to this and when Tanjiro is met with this he deflates a little.
However, it is later learned that the black sword is associated with the Sun Breathing technique which is the unnamed thirteenth breathing form and is theorized to be the first to which the other forms branched from.
Crimson Red
Bright red Nichirin swords differ from the others because they are not associated with any one of the breathing techniques. Instead, this state of the sword can be achieved by heating the blade and turning it red. This technique inhibits the regeneration abilities of demons. Sometimes when powerful enough burning them and leave permanent wounds.
This technique can only be achieved in three ways that are known, clashing katanas, Nezuko Kamado's Blood Demon Art, and raising the blade's temperature with a strong grip. However, in the two first stated situations, Demon Slayer Marks must be present for it to work.