So here's what I'm thinking about the "light green" RS3.
That green looks like the green they used on their last scheme (like the MP15s, SW1001s, GP39-2s and GP40-2s): http://www.readingmodeler.info/index.php/articles/89-locomotive-reference/road-locomotives/189-emd-gp39-2
My guess would be that they wanted to do a Reading unit and were afraid to tackle the green and yellow scheme and found a very late repainted RS3 as a prototype.
Or...
That they were trying to do the correct dark green and thanks to the wonders of old-school global communication before Webex just got the color wrong: "You said dark green boss".
I figured it out today!!!
Turns out I was right about there being an RS3 in the late all-green scheme. But just one.
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This is in the October edition of Railfan and Railroad.