... where the yellow on the ambulance is actally supposed to be brighter as well, if only the driver would keep the vehicle clean (too much soot on the back doors). The lighting of the photo isn't great either.
It might not be easy to achieve in game. Sure you can start with a 255/255/0 pure yellow on the basic color map, but the other texture layer might tune it down to this ugly pale olive (wow, did I just say "ugly olive" being a die hard fan of a military sim?^^ nevermind...). But don't give up until it looks like the signal vest you have in your car. There is probably a shader on it that was supposed to give the white car an ivory tone.
If you want to make it really great, try to make it reflective: http://upload.wikimedia.org/wikipedia/commons/0/0e/Ambulance_reflective.jpg
I have no idea how to write a shader for that, but wouldn't be surprised if it actually works well in Arma3.