def traffic_light_action(color: str) -> str:
    """
    Interpret a traffic light signal and return the action to take.

    The colour is recognised whatever its capitalisation, so 'red', 'Red',
    and 'RED' all mean the same signal.

    Rules
    -----
    - 'red': vehicles must stop, so return 'Stop'.
    - 'yellow': caution, so return 'Slow down'.
    - 'green': proceed if the way is clear, so return 'Go'.
    - Any other colour is invalid, so return 'Invalid color'.

    Parameters
    ----------
    color : str
        The colour of the traffic light, in any capitalisation.

    Returns
    -------
    str
        'Stop', 'Slow down', 'Go', or 'Invalid color'.

    Examples
    --------
    >>> traffic_light_action('red')
    'Stop'
    >>> traffic_light_action('YELLOW')
    'Slow down'
    >>> traffic_light_action('Green')
    'Go'
    >>> traffic_light_action('blue')
    'Invalid color'
    """
    raise NotImplementedError("Replace this line with your code.")