I've just had a wonderful discovery about why Power BI sometimes seems to choose random colors in the legend.
Typically, the first item in a series will match the first color of your Power BI theme, the second item in the series will match the second color of your Power BI theme, and so on.
However, this isn't always the case. I have noticed that sometimes when I have text category values for my legend that Power BI can assign random colors, seemingly not even part of my theme. Until recently, I just accepted this as a quirk of Power BI and carried on with my report development.
While developing a new Power BI theme JSON file and reading up on the nitty gritty details, I learned a few things that have helped me understand what's actually going on.
Dynamic Series
The first thing I learned about was "dynamic series". You can find the Microsoft Docs definition of dynamic series in small print under the very long theme article in the How Report Theme Colors Stick with Your Reports section.
What is a dynamic series?
To understand dynamic series, we need to understand what is meant by 'series'. Series is the fancy data viz term for items in the legend. There are two ways in Power BI visualizations to get more than one item in the legend:
- Add a column to the 'legend' property of a visual.
- Add more than one column to the values property of a visual (this could be called 'Values', 'X-axis', 'Y-axis', etc depending on which visual you're using).
When you use option 2, the number of items in the series will always equal the number of columns you added to the values property. This is a 'static series' because the number of items is static (meaning it doesn't change).
A dynamic series occurs when you use option 1. Depending on the filters and slicers in the report, the number of items in the list of values of the column used in the 'legend' property could change. Let's use months of the year as an example.
In the image above, we have added 'Month name' to the 'legend' property of the visual. So you can see we're using option 1 for creating more than one series in Power BI. This enables 'dynamic series'. We know there are always 12 months in the year, so it may seem like this is a static series. However, depending on the report filters and slicers, you may only see some of those months at a time.
As you can see in the image above, March is always red, even when February is missing. This helps ensure that color has meaning throughout your report and that even if you're only showing quarter 4 (October, November, December), they'll maintain the mauve, green and charcoal colors.
Themes have unlimited colors
The second thing I learned about the seemingly 'random' colors that Power BI applies to text categories / legend items is that a theme has more than the 8 colors you see in the Power BI desktop interface. If you use the JSON file to customize your theme, you can provide as many colors as you want (there's probably a limit but I haven't needed to find it yet). That means when you have a series with more than 8 items, Power BI will continue down the color list in the hidden JSON file until it runs out of colors. Then it will repeat.
Conclusion
So, if you see random colors popping up in your Power BI report, don't change them at the visual level. Figure out how many potential items are in your series and update the JSON theme to provide more color values if needed.
