Normally we use "red" or "orange".

MRinFLA; using a saftey word that sounds like, or really does mean "stop" isn't a good idea because it can be part of the fun to beg for the other person to stop what he's doing, just pretending that you don't like it.

If you pick a word that's really unlikely to come up in conversation while having kinky sex (like "pineapple") then you've got all the fun of "Oh, please, no, stop, please..." but you know your partner will know when you -really- want them to stop what they're doing.

Also, if you're in the middle of kinky sex it's handy to have a word that means "whoa, slow down there, I enjoy most of what you're doing, apart from -that-" because it'd kinda ruin the mood to have to have that whole conversation when "orange" or "yellow" works just as well and doesn't kill the moment.