Hi Adam,
By default each series will get a new color. And by default, when you have more series than the number of colors defined on a palette, additional shades will be auto generated.
The "Palette Custom Colors" QuickStart show show you can customize the palettes. You can also disable shade generation. This code shows how to use a single color for all the series:
<charts:XYChart Style="{StaticResource ChartStyle}">
<charts:XYChart.SeriesStyleSelector>
<charts:SeriesPaletteStyleSelector>
<charts:Palette IsShadeGenerationEnabled="False">
<Color>#BA2A67</Color>
</charts:Palette>
</charts:SeriesPaletteStyleSelector>
</charts:XYChart.SeriesStyleSelector>
...