Street lights that use high pressure sodium vapor produce light that is mainly yellow with some red why are dark blue police cars not advisable in a community that uses these street lights?

Answer 1

When illuminated with white sunlight, a dark blue police car reflects mostly dark blue - which you see - and absorbs all the rest, such as red, green etc. It is similar with any object of some color. If the light comes from a sodium light which is mostly just yellow and some red, there is basically no blue to reflect from the car which, in the spectrum between red and blue, reflects only blue. Therefore the car will appear black or rather very dark and hard to see.