Why can't I create an object with apparent type the type of its superclass?

Question

Why can I access a variable with private access without a getter (colour == ((ColouredPoint) other).colour)? Here is the code for my ColouredPoint class:

public class ColouredPoint extends Point {
    private Colour colour;

    public ColouredPoint(double x, double y, double z, Colour colour) {
        super(x, y, z);
        this.colour = colour;
    }

    @Override
    public String toString() {
        return "(" + super.toString() + ", " + colour + ")";
    }

    @Override
    public boolean equals(Object other) {
        if (other instanceof ColouredPoint) {
            return x == ((ColouredPoint) other).x
                    && y == ((ColouredPoint) other).y
                    && z == ((ColouredPoint) other).z
                    && colour == ((ColouredPoint) other).colour;
        }
        return false;
    }
}