1. Kimetsu no Yaiba Nichirin is gray
This is considered to represent the color of stone. In the movie Demonic Sword, the user of the gray sword is Gyomei Himejima. This is the backbone of the army of demon killers and is a fairly gentle normal person, but when necessary, it is also extremely scary and decisive. Among colors, gray is considered to represent two qualities, that is intelligence and stability. But both of these qualities coexist within Gyomei Himejima's body.
2. The yellow Nichirin demon-killing sword
Yellow is considered the color of thunder and the owner of the yellow Nigiri sword is Zenitsu Agatsuma. This character only shows his true strength when he is really cornered. At this moment, all of Zenitsu Agatsuma's inner strength and courage will suddenly explode.
The yellow Nichiri sword is characterized by a yellow sword and is emblazoned along the body of the sword.
3. Yellow Demon Slayer Nichirin
The black sword is the sword that Tanjiro Kamado possesses. Although not much is known about this colored sword, it is generally considered a sword with bad omen. However, there are also some theories that black is a symbolic color of coal and is very suitable for Tanjiro Kamado to use because he used to be a coal dealer. There are also some opinions that this color represents the sun.
No matter what it represents, this sword is quite unique in that it can absorb sunlight well - this is the point that demons fear most.
Above is some information about the color of the sword Nichiri in the movie Demonic Sword and the meaning of each color.