Hi @Hickling ,
Suppose you apply a custom color set (or individual color) to a particular data point in a visual by using the Custom color option in the color picker. When you apply a report theme, it does not override that customized data point color.
Or, suppose you want to manually set the color of a data point color by using the Theme colors section. When you apply a new report theme, those colors are not updated. To get your default colors back, so they'll update when you apply a new report theme, select Reset to default, or select a color from the Theme colors palette in the color picker.
Note: Many Power BI visuals won't apply to report themes.
For more details, please refer to:
Use report themes in Power BI Desktop - Power BI | Microsoft Docs
Beides, here is a similar thread that may helps you :
Data Colors Change in Service vs Desktop (differen... - Microsoft Power BI Community
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