It doesn't matter which one you buy. As long as they all are good cars, you will be ecstatic with your purchase. If it were me, I'd buy the Black with 12k miles on it and drive it as much as possible. You can wrack up those miles guilt free and the depreciation hit due to the addition of miles will be much lower.
I purposely searched out and bought my car, which had 16k miles on it, so I could enjoy it and drive, drive, drive. Three months after I purchased my 16k mile car I drove it from Vegas back to my home in Canmore, Alberta for a total of 1362 miles on that single trip. I made that trip with my wife and it was freakin epic! I didn't want a low mileage car because they cost a ton and they may have some mechanical issues due to sitting for 15 years (but typically those issues are an easy fix).
If you want to buy a GT, park it in the garage, stare at it and proclaim "how awesome the car is" then buy the silver one with 600 miles on it. You won't be wrong in your assessment. Don't drive it, put it on display and wax the shit out of it. It'll be a great investment over time.
If you, for some reason, decide to buy that silver one with 600 miles on it and subsequently drive the shit out of it, look me up and I'll buy you dinner, scotch, beers or whatever we can "cheers" over because you'll be "ok" in my books.
Just depends on what your objectives are when buying a GT. Ultimately it doesn't really matter, if it's a good car, you can't go wrong.
QSS