Danger signals are generally red as red light

The primary reason why the colour red is used for danger signals is that red light is scattered the least by air molecules. The effect of scattering is inversely related to the fourth power of the wavelength of a colour. Therefore blue which has the least wavelength of all the visible radiations is scattered the most and red which has the highest wavelength of all the colours we can see is scattered the least. So red light is able to travel the longest distance through fog, rain, and the alike. Also, red is a colour we inherently perceive as one that is associated with danger.