Rulings

• 2009-10-01
  Call to Arms's third ability is a "state trigger." Once a state trigger triggers, it won't trigger again as long as the ability is on the stack. If the ability is countered and the trigger condition is still true, it will immediately trigger again.
• 2004-10-04
  You can't choose "colorless" as a color.
• 2009-10-01
  Call to Arms affects all white creatures, not just white creatures you control.
• 2009-10-01
  If Call to Arms changes controllers, it will continue to check the nontoken permanents controlled by the player chosen as it entered the battlefield, even if that player isn't an opponent of its current controller.
• 2009-10-01
  The game continually counts the number of nontoken permanents of each color the chosen player controls. Multicolored ones are counted for each of their colors; colorless ones are ignored. The moment the number of nontoken permanents of the chosen color that player controls is less than or equal to the number of permanents of one of the other colors that player controls, Call to Arms's second ability stops giving white creatures +1/+1 and its third ability triggers. Call to Arms will be sacrificed when the third ability resolves, even if those numbers have changed by then.