def rgb(red, green, blue):
    """Helper function that returns rgb strings to be used
    by color-specification headers.

    Arguments:
        red (int): Red color value.
        green (int): Green color value.
        blue (int): Blue color value.

    Returns:
        A string representing the specified color.

    Example:
        >>> rgb(23, 167, 42)
        'rgb(23, 167, 42)'
    """
    return 'rgb({0:d},{1:d},{2:d})'.format(red, green, blue)