Q:

How do I access a private variable from a method?

currentColor = getCarColor(this.car.color)

Here color is private and getCarColor is a method, how do I access the variable color?

A:

You should not be accessing private variables directly: they are made private for a reason.
The proper way to do it is to add a public accessor method for the color to the car:
class Car {
    private Color color;
    // Add this method:
    public Color getColor() { return color; }
}

