def color_args(args, *indexes):
    """
    Color a list of arguments on particular indexes
    
    >>> c = color_args([None,'blue'], 1)
    >>> c.next()
    None
    >>> c.next()
    '0000FF'
    """
    for i,arg in enumerate(args):
        if i in indexes:
            yield lookup_color(arg)
        else:
            yield arg