Assertion : Colours are seen in thin layers of oil on the surface...

Colours are seen in thin layer of oil on the surface of water, because of interference of light. When white light falls on a thin film of a liquid it will appear bright having colour whose wavelength satisfies the relation $2 \mu t \cos r=(2 n+1) \lambda / 2$. The colour of the film will depend upon the thickness of the film and the angle of refraction.