Question

what will be the colour of cloudless sky in absence of atmosphere?

Answer 1

(a) In the absence of any atmosphere, there will be no scattering of sunlight and the sky will appear dark. (b) In the visible spectrum, the red colour has the largest wavelength. The red colouris least scattered by fog or dust particles. Therefore, we can observe red colour easily even in foggy and dusty conditions.

Answer 2

As we know, the colour of the sky appears to be blue is due to the atmospheric refraction caused by splitting up of sun's light.

In case, if there is no atmosphere than the colour of the sky will appear to be black since there is nothing which can refract the sun's rays.