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:
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.
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?
* 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.