def slope_line(first_point, second_point):
    """
    Calculate a value of a line slope for the given two points.

    Parameters
    ----------
    first_point, second_point : tuple
        A tuple containing xy coordinates (float) on the color-color plane.

    Returns
    -------
    slope : float
        A value of the slope line.
    """
    slope = second_point[1] - first_point[1]
    slope /= second_point[0] - first_point[0]
    return slope
