The danger signals installed at the top of tall buildings are red in colour. These can be easily seen from a distance because among all other colours, the red light:

The amount of scattering is inversely proportional to the fourth power of wavelength. And since the wavelength of red has the longest wavelength, the amount of scattering becomes smaller.

Danger signals installed at the top of tall buildings or traffic signals are red so that they

can be easily seen from a distance.