Fire engines used to be red. now many of them are yellow-green. why was the color changed?

Yes, that is true. Yellow and green colors are now the major colors of these fire engines due to our eye-sights. In the dark, red is one of the least visible colors so imagine what would happen if it was at night and you could barely see it. On the other hand, colors like yellow and green are spotted more easily and raise awareness in a proper and eye-catching way. 

I hope I was able to help you! :)