We drove out to Grandma Paula’s house tonight and while waiting at a stop light you asked me, “Why does red mean stop?”. I have no earthly idea. I couldn’t even come up with a reasonable guess of an answer. And that’s what I told you. I said, “I don’t know, but maybe we can investigate it, like Sid the Science Kid, when we get home!”
That seemed to please you for a second and then you said, “Maybe it’s because red is the first color in the rainbow.”
I did a little research when I got home and found that it mostly has to do with the railroads and red intrinsically meaning ‘danger’, but I think you’re idea is by far the best.