The speculations seem pretty much dead regarding the IS-3 seeing combat in World War Two. It was used in combat in Budapest during the Hungarian uprising in 1956, however. The Russian historians that Cookie Sewell is in touch with suggest FS 34102 as the closest green to the color standards issued during the war for Protective Green (though Steve Zaloga favors FS 34098 based on a Russian military part he owns). Based on a few color photos published in Tankmaster Magazine, I'd be inclined to favor the first (though it's always risky to base a color match on a photo). Take your pick.
There was a lot of color variation during the frantic days of the war, but presumably standards were tightened again during peacetime, particularly for a prestige vehicle like the IS-3 (which served much the same purpose in Moscow military parades that the T-35 had before the war). Both of the greens are available in most color lines as US Air Force cammo colors.

[thanks for sharing that info gerald. unfotunately, my paint options here in manila are rather limited (no model master, polyscale or testor products as far as I know). do you think i could achieve a reasonable match with the colors you suggested by mixing tamiya or gunze sangyo paints?