I have observed puddles of oil on the pavement with swirling colours more often on overcast days than sunny days. Does the phenonmenon of thin film interference work better with diffuse/scattered light than light coming directly from a source? Or, is it a question of light incident at an oblique angle rather than coming from directly overhead? OR, does it have less to do with the ambient lighting, and more to do with the rain, i.e. is there a thin layer of oil on top of a thin layer of water, adding additional reflective boundaries/optical media?