'Red' is used as a danger signal because:

According to Rayleigh's scattering law, the intensity of light scattered ($I$) by particles smaller than the wavelength of light is inversely proportional to the fourth power of its wavelength ($\lambda$):
$$I\propto \frac{1}{{\lambda }^4}$$

In the visible light spectrum, red light has the longest wavelength (${\lambda }_{\text{red}}$). Because of its large wavelength, red light is scattered the least by atmospheric gas molecules, dust, smoke, or fog. This low attenuation allows red light to travel long distances without significant loss of intensity, making it highly visible from far away as a warning sign.