Can a deck with a mono-colored commander have cards with other colors than its commander in it?

Question

The basic rules for building a Commander deck state:

All cards must be in the color identity of the commander

Then the rules summary continues with:

COLOR IDENTITY

A card's color identity can come from any part of that card, including its casting cost and any mana symbols in its text. Every card in your Commander deck must only use mana symbols that also appear on your commander. Colorless cards are allowed as well.

(emphasis mine)

So reading these rules makes it pretty clear to me, giving a "no" to my question. However, a couple of weeks ago I played a multiplayer game with more experienced players, but nobody made any remarks on the (winning) deck featuring a mono-white commander* leading a red-white deck. So now I'm confused. Am I missing something, or was that deck technically illegal?

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* I can't remember the exact card. It had doublestrike and was pretty cheap to cast, around 2-4 mana. It was not a flip-card, such as Archangel Avachyn in the example on the linked website.

Answer 1

As the rules you quoted say, the color identity of every card in a commander deck must be a subset of the color identity of the commander, where a card's color identity is the set of the colors of all mana symbols and color indicators in that card's rules text.

There are currently only three mono-white legendary creatures whose color identity contains both red and white:

• Archangel Avacyn - color ID WR
• General Tazri - color ID WUBRG
• Sisay, Weatherlight Captain - color ID WUBRG

The only other candidate that's close to your description and could have been valid is Sylvia Brightspear. She partners with Khorvath Brightflame, which is red. In the case of partner, a deck actually has two commanders, and they both contribute to color identity. It's possible Khorvath never got cast, or you simply didn't notice it was from the command zone when it was cast.

If none of the cases above are true, then your opponent's deck was not valid.

Answer 2

I have a mono white commander deck that plays all 5 colors, my commander is General Tazri. The reason why this works is because she has an ability:

: Ally creatures you control get +X/+X until end of turn, where X is the number of colors among those creatures.

That ability costs one mana of each color to cast, making her color identity contain all 5 colors, though her card's color does not.

Though the wording of the rule is poor, saying the mana symbols need to appear on your commander, the truth is that the commander must have the color identity of those symbols, which includes the color identity of the back of a flip commander, color setting text on the card (which is obsolete) and the color identity marker in cards that need it. There aren't many examples of these currently in legendary creatures, the only real ones to worry about currently are mana symbols in abilities and less common, color changing flip cards like Archangel Avacyn.

It's also worth noting that the color identity of the BACK of flip cards in the deck matters too, so a card like Elbrus, the Binding Blade which could be used in any deck based on the colorless equipment front side, still cannot be used in anything that's not black, because Withengar Unbound is black. This does not apply the same to mana symbols in reminder text, which is why cards with Extort can be in decks that are either black and white, as long as the card otherwise is only black or white, since the hybrid symbol is in the rules, not a part of the card.