def colon_hex(hexstr, interval) -> str:
    """ Convert hexstr to colon seperated string every interval

    :param hexstr: The hex string to convert.
    :param interval: The interval number.
    :return: The colon seperated string.
    """
    assert len(hexstr) % interval == 0
    return ':'.join(hexstr[i:i + interval] for i in range(0, len(hexstr), interval))