Well, this should kind of be in the You Make the Card forum, but it will get buried there quickly so I'm putting it here instead. Can you make a mono-colored instant-Trap card? Basically, a trap is an instant, with a mana cost, effect, etc. but you can play it for a much reduced mana cost if an opponent has played 2 or more creatures this turn. They are a cycle in Zendikar, but we've only seen the blue one. Green/Black/Red/White ones are preferable. Go!
Ben
