Many thanks for the responses, everyone.

Locked Advantages, it's interesting to know that there are couples who rely enough on other signals and dispense with a formal safeword. As long as a signal to stop is unmistakeable and does not get lost in the frenzy...

Ruby, I have never yet advanced to a point where I would need to decide in an instant: Is this possibly 'Oh no... But I leave it in your hands!', or is it 'No!' period. So far 'No' has always been 'No' for us too, and maybe it will always remain so. But I try to educate myself for all sorts of eventualities. I guess I am of a cautious and sceptical nature in this respect.
Yes, fiction is full of fanciful and not very practical safewords. That's one of the reasons why I posted this question, to see what people have found useful and practical. Although obviously if I ever introduced a formal safeword other than directly asking me to stop whatever I'm up to, it would not be in English.

Thank you bug and subthoughts. So the colour code does work indeed. LOL at the squeaky toy. Maybe I will need to go hunt for that dolphin I bought for the bathtub ages ago. whippedcream, I suppose that's the perfect way for a safeword to function: something you don't have to think about but which comes to mind immediately.

Djarumdoll, thanks for sharing your experiences. The thumbs down sign seems a good option – and it doesn't wake the neighbour's dog! Your reflections on trust, control and submission are interesting and thought-provoking. And I wholeheartedly agree with your approach of taking your time and listening closely. As my experience is limited so far, I think I could not dispense with an unmistakeable stop signal valid even outside an immediate emergency situation. My cautious nature again I suppose, which I need at the present stage.

Thank you all again for your contributions.