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Vigilance (Attacking doesn't cause this creature to tap.)
As Diamond Knight enters the battlefield, choose a color.
Whenever you cast a spell of the chosen color, put a +1/+1 counter on Diamond Knight.
Diamond Knight’s ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
If you somehow control a Diamond Knight for which no color was chosen, its last ability will never trigger.
You must choose one of the five colors of Magic as Diamond Knight asks you to choose a color. You can’t choose “artifact,” “colorless,” or “chartreuse.”
