Can City of Brass be used to cast Kozilek, the Great Distortion in Commander?
No. City of Brass adds one mana of any color, and colorless is not a color.


  105.4. If a player is asked to choose a color, he or she must choose one of the five colors. “Multicolored” is not a color. Neither is “colorless.”


In the past, if you attempted to add mana to your pool that did not share a color with your commander's color identity, that mana became colorless instead.


  Rule 4. If a player would produce mana of colors that are not in their General card's color identity, they instead get that much colorless mana.


This rule was removed with the release of Oath of the Gatewatch, which introduced new colorless symbols and colorless costs.