I noted a small oddity when photographing my early 90 passenger station with the yellow roof yesterday and decided to take a second look at the 93 freight station with yellow roof this morning to confirm the difference.
What I noticed was that the roof lip is crimped along the edge, instead of being bent over like all of the later versions I have seen. To me this definitely indicates the yellow roofed versions represent the earliest production.
Yellow Roof - crimped edge
Gray Roof - non crimped edge
These variations can drive one crazy!