The blue police cars would be difficult to see under sodium streetlamps.
Work Step by Step
Blue is the complementary color to yellow. Blue + yellow = white. Under normal illumination, blue cars absorb the complementary color to blue (which is yellow), reflecting blue light. The blue cars appear black when illuminated by yellow light, because the paint absorbs that light, and there is no "blue" light to reflect. This is discussed on pages 505-506 and 509-511.