In order to change the color of the shape you should change its Background but currently there is a bug in the Toolbox so you cannot directly set a value to the Background property and you should create a Style which sets this property and apply it to all shapes. Could you please try this approach and tell me if it works for you? Actually for the coming Q2 we will bring a brand new RadDiagramToolbox
control which you will be able to use in such scenarios.
the Telerik team
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