Monocolored means one color in respects to a card's or deck's color (i.e. white, blue, black, red or green).
From the glossary of the Comprehensive Rules (September 24, 2021—Innistrad: Midnight Hunt)
- Monocolored
- An object with exactly one color is monocolored. Colorless objects aren’t monocolored. See rule 105, “Colors,” and rule 202, “Mana Cost and Color.”
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