def _nt_colour(nt):
    """
    Set default colours for 21, 22 and 24 nt sRNAs
    :param nt: aligned read length (int)
    :return: colour code (str)
    """
    if nt == 21:
        col = '#00CC00'
    elif nt == 22:
        col = '#FF3399'
    elif nt == 24:
        col = '#3333FF'
    else:
        col = 'black'
    return col