I am not sure that I understand your scenario. Can you please elaborate more on how you load the items to the RadDiagramToolbox
control? If you create your custom collection then you are able to control the background color of the shapes that you add.
In case that you use our HierarchicalGalleryItemsCollection
you are allowed to change the color of the shapes in the RadDiagramToolbox
control by changing the theme of your application. Please take a look at the themes that we provide and see if some fits your requirements.
All the best,
Pavel R. Pavlov
the Telerik team