It's quite hidden in the docs, but here are the default values:
https://docs.splunk.com/Documentation/Splunk/latest/Viz/ChartConfigurationReference#General_chart_properties
Default value for charting.seriesColors:
[0x1e93c6, 0xf2b827, 0xd6563c, 0x6a5c9e, 0x31a35f, 0xed8440, 0x3863a0, 0xa2cc3e, 0xcc5068, 0x73427f, 0x11a88b, 0xea9600, 0x0e776d, 0xffb380, 0xaa3977, 0x91af27, 0x4453aa, 0x99712b, 0x553577, 0x97bc71, 0xd35c2d, 0x314d5b, 0x99962b, 0x844539, 0x00b290, 0xe2c188, 0xa34a41, 0x44416d, 0xe29847, 0x8c8910, 0x0b416d, 0x774772, 0x3d9988, 0xbdbd5e, 0x5f7396, 0x844539]
These apply to pie charts as well. If you want to define other colors in your pie chart, you can use the same options parameter.
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