I don't know how typical gray is for a tank, but I know that many Russian armored vehicles during World War I and the Civil War were painted with the same gray-blue paint that was used to paint warships.
The original A7V tanks were painted standard field gray when they were introduced into the army.
The picture you have attached is the vehicle "Wotan" from the tank museum in Munster Germany.
They have opted for this color sheme. (Again: Field grey could be almost every color between gray and green. There was no standard.)
I opted for a gray color, as it corresponds to the canon of wheeled armored cars used by the Kokampf.
Of course, the color is speculative. But since the two A7V tanks always look very dark in the pictures, a gray color is very likely.
But surely the two A7Vs could also be painted in a shade of green. It's really all very speculative. There is no real right or wrong.