There are a couple of approaches to solving this. Here's a link to an entire series devoted to custom/conditional colors in SSRS by Dave Lean:
Below is a technique mentioned in Dave's series that is quite useful:
Basically, you create custom code to handle the region; place a function call to the custom code in the relevant field property while passing the region value to the function.
Step 1: Paste this into the Custom Code window.
Public Function ColorRegion(ByVal RegionName As String) As String
Select Case RegionName
Case"Africa"
Return"Brown"
Case"Americas"
Return"Blue"
Case"ANZ"
Return"GoldenRod"
Case"Asia"
Return"Olive"
Case"Europe"
Return"MediumTurquoise"
Case Else
Return"Red"
End Select
End Function
Step 2: Use it in a report Expression to assign a value to a color property.
=code.ColorRegion(Fields!Region.Value)
Lastly, although you are creating pie charts, you may want to reconsider or persuade the person requesting them. Pie charts are generally not an effective visualization compared to bar charts when comparing values. Stephen Few (data visualization expert) has a website (www.perceptualedge.com) that should be required reading for anyone creating charts/graphs -- really great stuff. Here's a direct link to an article he wrote explaining the limitations of pie charts: http://www.perceptualedge.com/articles/visual_business_intelligence/save_the_pies_for_dessert.pdf