Thanks, Aircav. For all the times I've visited the Landships site, I hadn't searched the Forum.
Someone had asked the same question. "Mad Zeppelin" answered:
A7V 561: White name "Nixe" front right and rear left. Large white "V" front left and right, rear left and right. One Iron Cross per side. One Iron Cross on front louvre.
(Normal was one on front louvre and one on rear louvre, but 542 showed only the front one after capture. But 542 was not yet completely converted to the Abt.2 style. So, one may guess)
Colour of A7V, monochrome, either light grey or light green, or greenish grey or grey-green. Again, one may guess. (542 is depicted green in a model at the French tank museum that may have been made while eye-witnesses were still around.)
There's no evidence to support that the gun had a different colour than the rest of the tank, and the grey is only one possibility (that becomes more and more improbable with every new little piece of evidence that turns up).