Okay, after looking through aucfan (a YJA archive with 10 years of history), I'm now inclined to agree with suicider that there were in fact two different obi colors.
For example, see these auctions with the forest green obi:
https://aucview.aucfan.com/yahoo/h324363163/
https://aucview.aucfan.com/yahoo/j469062010/
https://aucview.aucfan.com/yahoo/w120662185/
https://aucview.aucfan.com/yahoo/e162150472/
https://aucview.aucfan.com/yahoo/r93456758/
And see these auctions with a distinctly lighter-colored lime green obi:
https://aucview.aucfan.com/yahoo/w123644610/
https://aucview.aucfan.com/yahoo/n113293654/
https://aucview.aucfan.com/yahoo/h434083039/
https://aucview.aucfan.com/yahoo/m81505883/
https://aucview.aucfan.com/yahoo/n140104692/
Now, it's possible that the lime green obis could just be faded. However, fading typically affects only part of the obi, e.g. the spine which is exposed to sunlight, leaving the other parts unscathed. In my experience it's very uncommon to see an obi that has perfectly uniform fading over all of its surfaces.
If you add to this that there are many examples of the lime green obi, I'd say it's almost certainly a real variant.
Any other thoughts from folks?
Note I sold my "lime green" copy a while ago, thinking it was just faded. Now I might need to pick up another copy of it
