Open the edit pane of the pie chart and switch to the advanced tab.
In the Mod section, select the “Advanced” option.
In the Layout options field, enter:
If it doesn’t work, you can also try:
Make sure you select the "Advanced" option.
To see other ways, you can have a look at this question: https://forum.mendix.com/link/space/widgets/questions/92564
If you're asking for the background of the parts of the pie chart,
Switch to the "Chart Properties" tab. You can add custom colors from the Colors section. These colors are added to the chart by sorting attribute.