The epoxy discoloring may be due to the exposure to the sun. I hear that most epoxies are sensitive to unltaviolet to one degree or other. In boats it's a common technique to seal and fill the wood with epoxy and then to coat it with 3 or 4 coats of marine varnish which has UV inhibitors in it in order to prevent the epoxy going cloudy or discoloring.

Or it may be that the epoxy is slightly pourous and is absorbing some of the color of the exhaust residue. Very few things we use are truly non pourous.