With color harmony in mind and using just a simple chart you can put more emphasis on the data shown. Using colors, you could help the viewer to distinguish the information easier as well as guide their eye to a certain area of a chart that you want to focus on.
Having a color theme in mind for my pie charts, I could use analogous colors that sit next to each other on the color wheel for a more harmonious color scheme.
Or I could use complementary colors that sit opposite each other in the color wheel for more contrast between each slice.
While it may look more appealing or even stylish to use analogous colors (it may be useful in some situations), for practical use I find it to be a better choice to use complementary colors. This way each slice has enough color contrast for the viewer to easily distinguish between each value that’s being displayed.