Just don't put too much value in safewords. We use the stoplight system as well, but mostly we just communicate during play. He can read my bodylanguage and the tone in my voice to guage exactly where I am.

And I don't always think of my safeword. Being pregnant, my current issue is nausea. And when it hits I don't think "red" I think "I am gonna be sick!!!" so that is what I say. Master then proceeds to get out of the way!

I am not saying that safewords don't have their place, they just aren't the be-all and end-all either.