When you see oil on the ground, sometimes it’s coloured. Why is this?

Question: When you see oil on the ground, sometimes it's coloured. Why is this?
- Keywords:
  coloured,
  ground,
  oil
Asked by SMILEY_FACE_:) to Hephzi, Imogen, Jen, Jennifer, Tom on 12 Mar 2015.
- Thomas Barrett answered on 12 Mar 2015:
  
  That is because oil is iridescent. Iridescence is the property of certain surfaces that appear to change colour as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, butterfly wings and sea shells, as well as certain minerals.

Question: When you see oil on the ground, sometimes it's coloured. Why is this?