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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 (July 23, 2021—Adventures in the Forgotten Realms)

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.”