I have the same problem as Gina. It's seems possible to change the colors of only the first ten slices.
I followed your advice and setted the number of point to a greater value. New slices appeared in the "Edit-Chart in Place", but it's impossible to customise them. The contextual menu appears, I can choose the colors, but the settings doesn't apply when used on any slide other than the first ten.
I attached a screen shot where I'm clicking on the blue slice, previously set to yellow. As you can see, the interface remembered that yellow was selected, but still draw the slice in blue. Every slice after the tenth share the last setting (here yellow) but are drawn with the official palette colors.
Any idea how to set more slices?
Do you think you could include a palette with only grayscales in it?