def get_stroke_color(xml_tree):
    """Determine the line color for annotations from a TCGA annotations file.

    Parameters
    ----------
    xml_tree : xml.etree.ElementTree
        The XML tree of the TCGA annotations file.

    Returns
    -------
    line_color : str
        A string representing the proper hex code to use for the stroke color.
        The output is always 6 characters and does not include the hashtag.
    """
    decimal_color = xml_tree.find('./Annotation').attrib['LineColor']
    line_color = hex(int(decimal_color)).replace('0x', '').zfill(6)
    return line_color
