The Merkava I is generally depicted as the old tan color in most representations, but the book on Mideast conflicts since 1973 shows IDF vehicles in greenish color described as khaki drab. So what's the truth?

The color during the 1967 Six-Say War was a tan sand color. Shortly before the 1973 Yom Kippur War, a dingy gray-green color was adopted, as documented by Egyptian researcher Vasko Barbic, who was allowed to examine Israeli wrecks in Egyptian Army junkyards. Of course, vehicles operating in the Middle East are usually covered by local dust, so original colors can be hard to distinguish in operational photos. Local IDF commanders were also known to modify issue paints to better match the terrain they had been assigned to.
Testors Model Master Sinai Gray matches this paint. It can also be approximated by mixing Model Master US Armor Sand and Olive Drab, 50/50. It would appear that this Sinai Gray was still the color standard when the IDF intervened in Lebanon in 1978.
I have no specific color matches for more recent paints. It looks like current colors are darker, but I defer to those with more information.

Very good matches for Merkava I, early 80's:

Humbrol Khaki Drill 72

50/50 mix of Model MAster SAC Bomber Tan and Header White

Since Humbrol paints are not readily available, I checked paint4models.com. They listed Testors SAC Bomber Tan as a substitute. Thanks!

And don't let the names fool you - both have that ever so slight greenish tinge talking heads used to tell us isn't there, but is.