I too pretty much stick with yellow for 'something's real-world amiss, check on me please' and red for 'full stop, untie me now, talk later'. I think they were the ones in the first story I ever read that was realistic enough include safewords and they just stayed in my brain. For non-verbal, I like this rubber sqeaky toy that is meant for dogs - it's shaped like a tiny cheeseburger, and I hold it throughout play -- squeeze it and it makes this goofy shrill sound. One squeak for yellow, multiple squeaks for red. It's loud enough to be very unmistakable. And it makes the neighbor's dog bark.