'Red' light is universally used as a danger signal because:

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

In the visible spectrum, red light possesses the longest wavelength. Because of its large wavelength, red light is scattered the least by micro-particles, smoke, dust, and air molecules in the atmosphere. This enables red light signals to travel long distances through fog or mist without fading or dispersing significantly, making them clearly visible from far away.