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

    Parameters
    ----------
    C : numeric
        Correlate of *chroma* :math:`C`.
    B_rw : numeric
        Ideal white *brightness* correlate :math:`B_{rw}`.

    Returns
    -------
    numeric
        Correlate of *colourfulness* :math:`M`.

    Examples
    --------
    >>> C = 0.013355007871688761
    >>> B_rw = 125.24353925846037
    >>> colourfulness_correlate(C, B_rw)  # doctest: +ELLIPSIS
    0.0167262...
    """

    M = C * B_rw / 100
    return M