The normal backcolor of the button is given by a FillPrimitive
which is using a Gradient, so that's the reason why you cannot just change the background color of the button.
To change this, you have to get the FillPrimitive and using the GradientStyle
property you can change the type of gradient (Gel, Glass, etc.), and the color to use.
(FillPrimitive))).BackColor = Color.Red;
(FillPrimitive))).GradientStyle = Telerik.WinControls.GradientStyles.Solid;
Keep in mind, applying a BackColor like that will override any setting to the BackColor coming from a theme, and also it will cause the OnHover state to be less visible.
Hope this helps, if you have any other questions or comments, please let me know,