If you are just doing something to the item, pass that in. This way the function doesn't need to know which array the item belongs to:
$scope.addColor = function(car) {
car.color = 'red';
};
If, on the other hand, you need to modify the array I prefer to pass in $index and save having to loop through the array looking for a match:
$scope.deleteCar = function(index, cars) {
cars.splice(index, 1);
};