And this from a page on ambulance visibility:
Green colours are located in the region of 490-560nm and yellow colours fall between 560-590nm.
During daylight conditions, human visual sensitivity peaks at 555nm, then shifting to 510nm when light is low. The reflectance values for lime-yellow paints (such as DuPont Imron 7741â„¢) peak at 550nm during daylight and between 520-530nm at night. These reflectance values almost exactly match the peak human sensitivity curve under all viewing conditions (both photopic and scotopic vision).
The eye resolves green-yellow better than any other colour. Green-yellow is unaffected by the Krovkov effect. The colour performs well in fog and under adverse weather conditions such as heavy rain and overcast skies. Green-yellow is virtually unknown as a colour in nature and is rare as a motor vehicle colour. It contrasts with almost every urban and rural background.
Solomon states (p71) that a comparison between DuPont Imron lime-yellow (7744)TM and Imron red (674)TM paints demonstrates that lime-yellow has a peak eye response approximately 4.9 times greater than red during daylight conditions. At night, the response is approximately 93 times greater, due to red being practicably invisible at night.
In a lateral angle situation when two vehicles are crossing an intersection at 90 degrees to each other, lime-yellow is detected significantly earlier than red, providing a longer time period to react and initiate an avoidance response.: