LOL. To be frank, you give the artist too much credit. I can almost guarantee you that he used a b/w photo as his model. This plus the VERY OLD myth about German late war stocks of dark grey paint being used gives us his interpretation -- thus, the grey/green Panther. However, it's verified what last production Panthers were painted from the factory.
There's another grey/green panther in the old Squadron/Signal Panther in Action book. Also if you look at old Tamiya catalogues, they have a dk grey Jagdtiger with yellow spots.
If you're gonna do the "what if" scheme, I'd do something completely out of the ordinary. If you paint a Panther grey/green, you'll get tons of critical comments:"Didn't that guy know that it was a myth?" and immediately turn folks off. If you come with say, an urban scheme like the BOAR Centurions, then it could be really cool!
But to answer your orig question: late war Panthers were factory painted with a basecoat of dk green. Swaths of dk yellow and red-brown were sprayed in cloudy patterns. (At least from MAN factory)