class KanbanColor(models.Model):
    _inherit = 'crm.lead'

    def update_kanban_color(self):
        curr_time = datetime.datetime.now()
        scheduler_line_obj = self.env['crm.lead']
        scheduler_line_ids = self.env['crm.lead'].search([])

        for scheduler_line_id in scheduler_line_ids:
            scheduler_line = scheduler_line_obj.browse(scheduler_line_id)
            last_update = scheduler_line.__last_update
            _logger.info(last_update)
            scheduler_line_obj.write(scheduler_line_id, {'color': 1})


ValueError: <class 'AttributeError'>: "'crm.lead' object has no attribute '_KanbanColor__last_update'" while evaluating
'model.update_kanban_color()'

What i do wrong? 
And how to access __last_update field?