Landships II

The Mk.IV: Most probably chocoloate brown all over, white Number 4066 on front of cab and on rear of side armour, nothing else. (I've no picture, I'm only best guessing.)
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.)

Both pictures are artist's impressions of the Munster tank museum replica, which is not Wotan and neither Nixe, but 506 "Mephisto" fitted with a socle mount and a 542 "Elfriede" turret. Both indulge on the Munster camo scheme which is light grey with a mustard gun and gun shield.
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).
The left picture, however, gives a quite good impression of what "Nixe" may have looked like (if you blend out the colours).

Paperpanzer, your "Nixe" has the wrong louvres. 561 was 2nd lot, so no 45 small slots but 9 long slits.

2nd lot louvre

2nd lot louvres: 525, 526, 527, 528, 529, 560, 561, 562, 562, 564