That's an interesting point about fire trucks... I agree with your overall point, to be sure, about the association between color and object. What you've reminded me of is that the use of red for emergency and alert signals isn't arbitrary. There's something about red that the human system responds to more vigorously than other colors.
I really have no idea why... physically, of course, red is the longest wavelength of visible light, but that makes it the least energetic visible wave. Red is alarming, green is relaxing, blue is revolting (scientists did an experiment with trick lighting; when the light changed, and people discovered they were eating blue-colored food, even though it had only been colored with food coloring, they puked). Red... blood? Green... grass? Blue... mold? From an evolutionary context, there would be advantage to reacting in those ways to blood, or leaves, or moldy food.
The problem that sticks, from this perspective, is that the evolutionary development for sound is for changes in sound-- not for static pitches. The evolutionary arguments for relative pitch seem perfectly reasonable; a particular interval of sound tells you that an object (predator or prey) has moved from here to there, and at what speed; hearing a certain dissonance or consonance in a person's speech, relative to their tonic, tells you exactly how they feel... so you know how to respond to them.
Is there an evolutionary reason to draw meaning from pitches? Anything at all? Some animals have evidenced absolute pitch, even now... I haven't read those papers because I never much cared about nonhuman absolute pitch... but what good does it do them to have it? What does it tell them? (Or are the scientists wrongly interpreting animals' behavior as AP listening, even as Jenny Saffran's research makes specious AP claims of babies?)