O.K. here are my thoughts.
I'll comment about the colors.
Merkava tanks when fresh from factory might have different colors between turret and cannon sleeve. After returning to the shop and repainting, the sleeve (made from bresent) around the cannon should be the same color as the turret.
The most I've seen are painted this way. it is correct as well to the sleeve of the periscope, forward to the loader/gunner hatch.
As well as the commander's scope.
The Loader/gunner hatch (and all the hatches) shold'nt be black. they shoud be turret color (unless you dont plan to display open hatches it's O.K.)
Ammo racks shouldnt be green. I dont remmember seeing one like that. Correct me if I'm wrong guys.
It should be a mediume grey or a light grey. sometimes
dark green (lets say somthin around humbrol 30, sometimes as dark as 92)
Add lots of washes. Merkava's that been at the desert are getting finly grind sand on them that makes them look with sand colour.
if you want to replicate this use a lot of pastel chalks.
If you want a northern merkava. keep it "cleaner" then the southern one but do use mud (depending on the season).
Dont forget the flat coat at the end.
Good luck!