Most vehicles would have been plain Olive Drab from the factory.
However, some units did camo paint their vehicles, generally in response to local conditions, time on the ground and local commanders orders or after field modifications had been fitted.
4th Marine Tank Battalion had a three (or even four) tone Sand/Green/Brown scheme, that they sported on Iwo Jima.
5th Marine Tank Battalion seem to have used a Green/Brown camo on some vehicles, also on Iwo Jima.
3rd USMC when landing on Guam had some three tone camo painted LVTs.