I think they were all in French colors, because we got them from France. I don't believe any US made FT 17s made it to France before the end of the war, if at all. But weather they were in French gray or camouflaged, I don't know for sure. I'm sure there are exceptions to the rule.
VR,

Nick,
Well, I've scoured my resources, notably Armored Fighting Vehicles Vol. 1 by Profile Pubs., Tanks and Weapons of WWI, Squadron's FT17 in Action, and Osprey's Meuse-Argonne and St. Mihel. After pouring over photos and captions in those books, it appears most if not all US tanks had some form of camouflage. Patton's 327th tank Regiment at St. Mihel appears like most tanks had three color camo applied-- a red-brown, green and cream colored camo (according to the example at the Patton Museum and in Armored Fighting Vehicls vol 1, and Squadron pubs book). The first French Renault built tanks initially came from the factory in a "French gray" color, which was a dark gray with a blue tinge. But it also appears that many came from the factory painted in camouflage too. There were some variations in the camouflage scheme too-- some tanks had red-brown, cream yellow (or tan), green and gray angular patches with the division between colors edged in thin black lines while others had just the red-brown, green and light tan/cream colors applied either by hand or airgun-- maybe it was dependent on what manufacturing plant produced the tank. It appears some of the red brown/green/cream-sand tanks had thier camoflage randomly spray painted while others were definitely hand painted-- perhaps in the field. And some tanks may have only had brown and green colors applied. I couldn't find a single photo of a US tank without camo of some form. (Until post war tanks appeared anyway-- which look like OD green). In many cases where it could be identified, there was a large outlined serial number with the base color showing through, painted below the turret on the hull sides-- but there were exceptions too--some with no discernible serials or a smaller solid serial stencil. Hope this helps, but I think you won't go wrong if you use camo-- I suspect the information on this subject is one of those items "lost to the ages".
VR, Russ