Units are assigned contingency areas where they will operate in war time. When new vehicles are rolling down the assembly line, if the overall unit is know (say 1st Cavarly Division) the vehicle will be painted for that contingency area, which for 1CD is tan. Pre 2001, all vehciles that werent going directly to units (depot for instance) were painted CARC 383 Green. When the vehicles were issued out to units, they would either stay green, get the black and brown CARC 383 added, or get painted tan. Every division has a paint shop where they paint vehicles. All NG/USAR units have some higher level maintaince unit that has paint capability somewhere in their state or region. Sometimes painting a vehicle is very low priority so it is very possible a unit is supposed to have 3 color camo, but the all green vehicle might stay that way for it's time at that unit. Other times, like in Active Divisions, a vehicle's first stop is the paint booth to get it's contingency color if it is not alrready.
And then there are always exceptions.. My ex USAF M35A2C we Green from the factory, then was painted 3 color camo CARC by a local Ford Dealership.. I have the paperwork and reciepts saying so!
A a rule of thumb for a unit that has tan vehicles, you could always tell how well their maintaince program was, as all new parts came in green. If you saw lots of green parts, stuff was generally well maintained. If it was all tan, maintaince could be lacking... Thats how my Division Commander did spot checks of motorpools as he rode thru them.....