A couple of things you might try.
First, check out a book "Gas Stations" by Wayne Henderson and Scott Benjamin. It has lots of Esso photos (black & white), many of them are credited to Chip Flohe. You may be able to locate him and possibly some color original photos.
Second, try several good automotive paint supply houses, ask to look at their old (tell them -real old) "fleet" color paint books or lists. You may have to try and try again before you find one that has some old books or is willing to go to the trouble to bring them out for you. These old books are like gold so they are not allways easy to get to. This should give you a good list of the colors used.
If you can get the color codes and a color photo you should be well on your way.