It strikes me that this could be quite useful when used proactively.
E.g. - Player Character Byrnwolf just got a crit on the Bandit Lord with his Greataxe. Queue the GM: "How comfortable would everybody be with a description of Byrnwolf splitting this guy in half?" All the players tap red, yellow, or green in response, and the GM can moderate from there.
It would be good at providing ongoing feedback for the GM to guide their style over time.
So another way for people to say “I don’t like where this is going?” What’s wrong with saying “I don’t like where this is going?”
Nothing is wrong with just saying it. In practice, it sometimes doesn't work out though.
For a very public drastic example, look at the Far Verona rape:
The reaction of the other players at the table while the scene plays out is telling. It appears that no one expected this storyline to go where it went.
Yet, nobody said "I don't like where this is going."
To be clear: I don't blame them for not saying it. Probably, I probably would have been quiet in that situation too. I believe that safety/communication tools are usually not necessary but in rare cases they are. Thus, it is a good practice in general and worth some overhead.