RadRibbonBarGroup consists of two FillPrimitives (in addition to the main FillPrimitive). These fills should be exposed to the user, so that s\he can tell what their colors should be.

Resolution: 
RadRibbonBarGroup consists of two FillPrimitives which are exposed: GroupFill and BodyFill. You can set their color using the following code snippet: 
radRibbonBarGroup1.BodyFill.BackColor = Color.MediumPurple;
radRibbonBarGroup1.GroupFill.BackColor = Color.MediumSeaGreen;