$(a)$ What will be the colour of the sky in the absence of an atmosphere?
$(b)$ Why are traffic light signals (or danger signals) of red colour?

(N/A) In the absence of an atmosphere, there is no scattering of sunlight by air molecules or dust particles. Consequently, no light reaches the observer's eye from the sky, making it appear dark.
$(b)$ According to Rayleigh scattering, the intensity of scattered light is inversely proportional to the fourth power of its wavelength $(I \propto 1/\lambda^4)$. Red light has the longest wavelength in the visible spectrum, meaning it undergoes the least scattering. This allows red light to travel longer distances without being significantly dispersed, making it clearly visible even from a distance through fog, smoke, or dust.