Map of List: Sum all the values of the list class

Question

I have this class

public Car(Colour colour, int passengers) {
    this.colour = colour;
    this.passengers = passengers;
}

Where Colour is an enum:

   public enum Colour {
    RED, YELLOW, GREEN, BLACK, WHITE, BLUE, GREY
}

I have created this map:

Map<Colour, List<Car>> colourListMap = new HashMap<>();

Imagine we populate the map of lists:

Car yellowCar = new Car(Colour.YELLOW, 4);
Car blueCar = new Car(Colour.BLUE, 1);
Car blackCar = new Car(Colour.BLACK, 3);

Map<Colour, List<Car>> map = carMemory.addCarByColour(yellowCar);
carMemory.addCarByColour(blueCar);
carMemory.addCarByColour(blackCar);

As well with 2 instances of the same colour:

Car redCar = new Car(Colour.RED, 2);
Car redCar2 = new Car(Colour.RED, 6);
Map<Colour, List<Car>> map = carMemory.addCarByColour(redCar);
carMemory.addCarByColour(redCar2);

Is there a simple way to sum all the passengers from the Car class? Instead to call the key by Colour? I was thinking to Java 8 Stream... but I m not very confident.

I know this isn't correct.

int size =  map.values()
                .stream()
                .mapToInt(Collection::size)
                .sum();