def METHOD_NAME(self, hex_value, amount):
    """
    Change the saturation of a colour by a relative amount (-1 to 1) and
    return the new colour hex value.
    Params:
        hex_value   (str)   Hex value
        amount      (float) Relative amount to saturate (-1 to 1)
    """
    c = colour.Color(hex_value)
    # Value must be between 0 and 1
    new_value = c.get_saturation() + amount
    if new_value < 0:
        new_value = 0
    elif new_value > 1:
        new_value = 1
    c.set_saturation(new_value)
    return c.get_hex_l()