def __insert_color(txt, s, c):
    """insert HTML span style into txt. The span will change the color of the
    text located between s[0] and s[1]:
    txt: txt to be modified
    s: span of where to insert tag
    c: color to set the span to"""
    return txt[:s[0]] + '<span style="color: {0};">'.format(c) + txt[s[0]:s[1]
        ] + '</span>' + txt[s[1]:]
