def colourfulness_correlate(C, F_L):
    """
    Returns the *colourfulness* correlate :math:`M`.

    Parameters
    ----------
    C : numeric
        *Chroma* correlate :math:`C`.
    F_L : numeric
        *Luminance* level adaptation factor :math:`F_L`.

    Returns
    -------
    numeric
        *Colourfulness* correlate :math:`M`.

    Examples
    --------
    >>> C = 0.104707757171
    >>> F_L = 1.16754446415
    >>> colourfulness_correlate(C, F_L)  # doctest: +ELLIPSIS
    0.1088421...
    """

    M = C * F_L ** 0.25
    return M