I have worked over the requirements shared by you and like to share that it seems to be issue in Aspose.Slides while setting the background color for SmartArt shape. I have created an issue with ID SLIDESNET-33195 to further investigate and resolve the issue. This thread has been linked with the issue so that you may be automatically notified once the issue will be resolved. You have to use the following code snippet for setting the SmartArt background color when issue will be resolved.
DiagramEx sArt = (DiagramEx)pres.Slides.Shapes;
sArt.FillFormat.FillType = FillTypeEx.Solid;
sArt.FillFormat.SolidFillColor.Color = Color.Red;
We are sorry for your inconvenience,
The issues you have found earlier (filed as SLIDESNET-33195) have been fixed in this update.
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