When something breaks in production, stop everything, find the root cause, fix it, then restart. A red signal alerts the team to problems so they can investigate and resolve issues before continuing work.
One of the things that we learned from lean manufacturing is when you have a problem, stop the process, understand why, address it, and then restart. And I think that the red is just another signal for the business as a whole to understand what's happening and fix it and then move on, but to acknowledge that that problem exists in the first place so that it can be properly addressed.
