parse a string of acceptable colors into matplotlib, that is, either
       strings, or three tuples of rgb. Don't quote strings.

def parse_ctuple(s,length=2):
    '''parse a string of acceptable colors into matplotlib, that is, either
       strings, or three tuples of rgb. Don't quote strings.
    '''
    if parse_utuple(s, colrx_s, length=length) is None:
        raise ValueError("{} is not a valid color tuple.".format(s));
    #quote strings
    s=quote_subs(s,colorfix=True);
    return evalt(s);