There are certain circumstances where Power BI will change the legend (and data) colors. One example is when your visual is created using streaming data, a new month begins, and a new category is introduced into your visual. Let's say that you've set the data colors for all five categories in the line chart above. And now it's Jan-13 and another manufacturer has entered the market. Because you did not set a data color for that new manufacturer, you may find that Power BI has changed the data colors for the original five manufacturers. When a new category is introduced, you may have to reassign data colors to the new and existing categories using the Formatting > Data colors pane.