I only establish safewords for CNC sessions where "no" doesn't mean "no". Otherwise the safeword is "oh no, that hurts too much."

I much prefer more nuanced communication over some red/yellow system, which always feels a little clunky to me. If a sub is getting close to his limits I'd rather know the specifics, like "Mistress, these nipple clamps are getting to be too much" so I know to slack off on his nips while continuing with the ballbusting or whatever.