In Commander, can you run a card with a hybrid mana cost if only one of the hybrid colors is in your commander's color identity?

This is actually the opposite of what many players expect: a hybrid mana symbol like {W/U} contributes both white and blue to a card's color identity (CR 903.4, 202.2c). This means a card with {W/U} in its mana cost has a color identity of white and blue.

For a card to be legal in your Commander deck, your commander's color identity must contain all colors in the card's color identity (CR 903.5b). Because a hybrid card's identity includes both colors, your commander must have both colors in its identity — not just one. So if your commander is mono-white, a card with {W/U} is actually not legal, even though you could theoretically pay its cost with only white mana.

A common misconception is that because you can pay a hybrid cost using only one color, only that one color matters for color identity. The rules explicitly reject this: the symbol itself defines the identity regardless of how you pay for it (CR 202.2c).

Example: You run Atraxa, Praetors' Voice (white/blue/black/green) as your commander. A card costing {W/U} is legal because Atraxa's identity includes both white and blue. But if your commander were Heliod, God of the Sun (mono-white), that same {W/U} card is illegal — blue is part of the card's color identity, and Heliod doesn't cover it.

Bottom line: hybrid mana symbols require your commander to have both colors of the hybrid symbol in their color identity, not just one.