Say my commander is {krenko, mob boss}. Can I have {izzet key rune} in my deck. I don't think so because you have to pay blue for its ability, but my friends think you can because it doesn't require colored mana to play. Who's right?
You're right. It can't have an activated ability that requires anything besides red mana.
You are even though it's an colorless it, can tap for blue which makes its blue/ red
In response to this question as a follow up, my mate has {Pontiff of Blight} in his Grixis Zombie deck. Is that a deck-legal card, seeing as Extort uses either Black or White? His Commander is {Crosis, the Purger}.
Pontiff has a colored symbol in his REMINDER text, not rules text. It is legal, since his color identity is completely black
Cool. I'll let him know.