Actually, it is more than 6 colors. The colors change but it is subtle with the default color scheme. If you choose a different color scheme than the standard one, the colors are more obviously different. I forget how many colors it has before it repeats. 16 I think.
You can change the colors of the wedges manually. You can save your new color scheme as a chart style. Move the new style to the first position and it will be the default. If you want to have that style in new documents, create it in whatever template you use for new documents and save it as a template. You can also get it into other documents by copy/pasting the modified chart to a new document then creating a chart style from it in that document.