I'm currently implementing the Google Visualization API into an existing platform, one requirement of which is the ability to choose custom colours for chart elements (such as pie slices and columns).
There is some consternation over whether to provide colour palettes as groups of similar colours to match the company branding (#138181) or to provide colour palettes of disparate colours such as those in the example links below.
I'm of the view that using disparate colours is easier to comprehend than similar colours, particularly when number of slices > 4. I have only ever seen similar colours used in the professional media when the maximum number of slices is around 5.
The kicker is that generated charts will have between 1 and 20 slices (pie slices, columns, or dot arrays in the case of scatterplots).