The importance of attention in the observation and perception process

What caught your attention in the previous picture? Taxis? Most would say, yes. Did anyone notice a red truck? Some people probably did not notice it. How to explain this? Here, not only deepening into optics will help, but also psychology. It has been noticed that red vehicles are less noticeable than the yellow ones. This explains why in some cities fire trucks are painted yellow, giving up the traditional red colour, or why some new ambulances are yellow.

Source:MZACHA (rgbstock.com)
http://www.rgbstock.com/photo/nypaxlU/Ambulance