Currently, Power View is limited in color customization, so getting specific colors will be an issue. Also, color must be a discrete value from a column and can't be a measure. This means that you can't really make the color change dynamically
based on slicer selections.
Without knowing the details of your data model it is hard to get specific, but if you don't need to filter the Power View Map, you need to create a calculated column that marks each city as Best or Worst, then use this new column in the Color field.
If the entire dataset for a given city is marked as either best or worst then the pie chart will only show a single value and a single color.
Again this isn't dynamic so if you require the data to be filtered, the best/worst banding will not change for each city when you start filtering the data.
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