I have attached a very simple project for your reference. The project does not have the UseAutoGeneratedItems property set to false. I have just attached it to show you that an output similar to yours is possible without manually adding legend items.
Can you explain what is your scenario? Why do you need to add the legend items manually? Is it possible that you use the approach in the attached project?
If you demand to add items in the legend manually - you might need to define chart palette brushes. The chart will use these brushes for the bars and you can use these brushes for the legend items.