Going further I have a bit more advanced solution in mind that will let us disable randomization for specific vehicle classes only.
Mission designer will define something like:
BIS_noVehicleRandomization = ["Offroad_Base"];
And all current and future init randomization scripts have to be wrapped with
if(({_this isKindOf _x} count (missionNamespace getVariable ["BIS_noVehicleRandomization", []])) == 0) then {
//Randomization code here
};
This way vehicles of Offroad_Base class will not get any kind of color or details randomization while other will.