Gaming is for having fun. Fun requires clear rules.
Only blue and green cards may be "in play".
brown-bordered cards (= play and discard)
which means that this card doesn't stay in play.
And your ruling would mean that
you can play only 1 BANG! card per turn
Blue cards in front of the players are hence defined to be “in play”.
Green bordered cards in front of the players are also defined to be “in
But note that most green cards have only 1 copy in deck.
Remember that it's a limit only for a player, so many players may a have e.g. a Barrel in the same time.