Colorless

Colorless has two in-game meanings.

1. A type of mana.
2. An object with no color is colorless. Colorless is not a color.

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 107.4c The colorless mana symbol {C} is used to represent one colorless mana, and also to represent a cost that can be paid only with one colorless mana.

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 202.2b Objects with no colored mana symbols in their mana costs are colorless.