From what I've read from the Unofficial Kimetsu no Yaiba Wiki, the Nichirin blade is often associated with the breathing style a user holds; I'll provide the chart below. I'll also suggest when they should be handed to someone and what their requirements should be.
Nichirin Blade Colors by Breathing Style:
- Sun = Black
- Moon = Light Purple
- Water = Blue
- Flame = Red
- Thunder = Yellow
- Wind = Green
- Stone = Gray
- Flower = Light Pink
- Insect = Lavender-Blue
- Serpent = Lavender
- Love = Dark Pink
- Sound = Amber
- Beast = Indigo-Gray
However, I don't think this is how it really works. Instead, I'll suggest an alternative system for how this can work in your game or story. Since Sun and Moon breathing aren't easy-to-access or aren't found in-game, consider the following suggestions:
-
Player Choice or Clan Influence:
- Allow the player to choose the color of their Nichirin sword. This could be based on the type of clan they were born into, adding a layer of lore and personalization.
-
Material-Based Color:
- Let the player choose the material the Nichirin Sword will be forged with, where each material gives the sword a specific color.
-
Keep it as the Demon Slayer Series:
- The color of the Nichirin sword changes to match the breathing style the user is wielding.
Requirements for Receiving a Nichirin Sword:
-
Breathing Style Mastery:
- The player must have a breathing style and have mastered it to a certain degree.
-
Selection Process:
- The player must have passed through a selection process.
Considerations for Implementation:
- Color Influence on Abilities:
- Each color of Nichirin blade could offer unique abilities or buffs related to the breathing style.
- Example: A blue Nichirin blade might enhance water-based attacks, while a red one boosts fire-based techniques.