Question

why are the danger signals mostly red coloured?

Answer 1

Longer the wavelength, lesser will be the scattering of light. Now, among the colors of visible light, red has the longest wavelength. Hence, it is least scattered by the atmospheric particles. As a result, whether it is fog or smoke, red light passes comparatively easily through them.

Answer 2

Because red colour has the maximum wavelength and less deviation as well as least frequency.

Due to maximum wavelength, we can see the red colour very easily even when we are very far away. That's why danger signals are red coloured.