I truly hope your going to stick with painting it to look like its from the Gundam universe. So many people on these forums will take one of those hard graph kits and then re-purpose it as something other then what it is, its disappointing.
As far as Caliber types you have three choices for the time period those tanks REALLY saw action.
The Zaku I which use 105mm rounds
the Zaku II which used 120mm rounds
and the Magellan Attack Tank which had a 175mm cannon and three 35mm machine guns.
also during that time period zeon used soft skinned armor cars that were amphibious for scouting, they are quite similar to the German 222 in that it has a low profile turret with a either small yield cannon or AA gun.
edit
This is the standard paint scheme for these tanks when they were used in desert regions
You can also go with the simplified version that was seen in the anime, which was a bluish purple color all over but I dont suggest it.
All in all when this vehicle was displayed and had any kind of pronominal roll in the series they had moved past the basic anime color and the Federation tool on the colorings of a real army. So I suggest you go to real modern tanks for inspiration of color scheme.