We have seven colours — violet, indigo, blue, green, yellow, orange and red (Roy G. Biv). Our atmosphere has a number of particles and when light falls on them, it gets scattered. With blue colour having less wavelength and more scattering qualities, it scatters and makes the sky blue. While red colour has opposite qualities than blue so traffic lights are of this colour.

His observations on the "strange events in our solar system" and as to why the sky looked blue and red colour was used in traffic lights to signal to vehicles to stop. - When Prof Jayant Narlikar saw the sun rise in the west