def node_to_col(node, num_rows):
    """Returns column ID of a node in the intance's graph representation that represents a column.
    
    Args:
        node (int): The node ID of the column in the graph representation.
        num_rows (int): The number of rows in the instance.
    
    Returns:
        int: Column ID of the node. If negative, the node is not a column but a row.
    """
    return node - num_rows
