Each player who has cast a nonartifact spell this turn can't cast additional nonartifact spells.
10/1/2008 The only difference between a colored artifact and a colorless artifact is, obviously, its color. Unlike most artifacts, a colored artifact requires colored mana to cast. Also unlike most artifacts, a colored artifact has a color in all zones. It will interact with cards that care about color. Other than that, a colored artifact behaves just like any other artifact. It will interact as normal with any card that cares about artifacts, such as Shatter or Arcbound Ravager.
10/1/2008 In other words: Each turn, each player can cast any number of artifact spells plus a maximum of one nonartifact spell.
10/1/2008 This effect counts all nonartifact spells that are cast, even those that are countered.
10/1/2008 This effect takes into account spells that were cast earlier in the turn that Ethersworn Canonist entered the battlefield, including any spells still on the stack. However, any spells on the stack as Ethersworn Canonist enters the battlefield have already been cast by that point, so they’re not affected by it.