I'm doing one of the original Sherman M50 vehicles based on the M4A4 chassis. Can anyone tell me what color these vehicles would have been painted?

A sand color is probably most appropriate. Here is a restored M50 (museum piece I assume) off Wikipedia:

I suppose it depends on how original. When the conversions were made with French assistance in the 1950's, the tanks were delivered in French Olive Drab. At that point, they would likely have had the Continental radial engine (nobody liked the M4A4's original Chrysler multibank powerplant, and many postwar users swapped other Sherman engine packages--the Israelis favored the radial, while the Egyptians favored the twin diesels from the M4A2).
In the early 1960's, Israeli tanks were repainted a warm sand color, and the M50's were rebuilt with Cummins diesels. Shortly before the 1973 war, the Israelis switched to a dingy gray-green color (this is the Model Master "Sinai Gray" paint color). The engine deck and exhaust for the Cummins was also updated in the 70's on the M50 and M51, though it was no longer a front line vehicle.
IDF paint has become darker in recent decades, and current Merkava IV's are nearly olive in hue, though the color is usually lightened by the ever present dust.

Matt, Gerald: Thanks for the quick replies. This project just got a LOT easier.

If you are going to model a tank with the original VVSS, then, I believe, it would almost certainly be the French Olive color. I think the sand color came out around the same time that the tanks were being upgraded to HVSS and Cummins engines. I haven't seen a single photo of an M50 with VVSS that was painted the sand color except for the rare gate guard.

Leroy: I've looked for a color match for French olive drab paint and can't find a thing. what brand would you recomend? I plan on doing a very early M50.

On this subject, Lifecolor showed a new IDF paint and pigment set at the Nurnberg Toy Fair. It looks like it will be quite useful to IDF fans - New Lifecolor IDF Set

Well, that's a tough one. I like the idea of using the new LifeColor paint but have never tried that brand before so can't offer comment. I think French Olive had a bit of a brown tint to it. I'd probably go with Humbrol 155 or something similar. I think OD is pretty forgiving especially if you intend to do a bunch of weathering....
The web page: http://idfmodelling.free.fr/article01.html lists a couple of alternatives...
If you can, check out "Lion and Lioness of the Line vol 1" by Robert Manasherob. It is mostly about the early M50 program and full of great photographs. It was reviewed here on Armorama. It is a great resource.

I hope you post some photos as you go so I can get inspired to do my M50. I too want to do an early one....