I am sure it is a bit late for you but this post shows up #1 in google so I am posting this here. I had to overcome this problem recently and found a solution. You need to edit the source of our web part page in designer and find the "PaletteCustomColors" attribute for the chart XML and change the values there to the colors you want to use in your chart.
I wrote a more in-depth method on my blog here: http://matthewchurilla.blogspot.com/2013/02/setting-custom-color-palette-on-chart.html
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