Assertion (A): Danger signals are of red colour.

Reason (R): The velocity of red light in the air is maximum and thus, has more visibility in the dark. 

Different components of white light have different wavelengths and frequencies. The scattering of light depends on the wavelength of that light. The longer the wavelength, the less the scatter will be. 

The red colour is always used for danger signals because, as the red colour has the longest wavelength, it is the least scattered. Therefore, the signal can be seen even from a very long distance.