Yeah, as far as I know, you need to define a customized theme to customize pie chart colors. You can prepend the chart with initialization logic like:

%%{init: {"theme": "base", "themeVariables": { "pie1": "#FF5733", "pie2": "#33FF57", "pie3": "#3357FF", "pieStrokeColor": "#000000", "pieStrokeWidth": 3, "pieOpacity": 0.8 }}}%%

This looks like it works on this site too.

To be fair, pie charts are horrible in general.