By design, the application menu in RadRibbonBar has a RadImageShape applied for the different elements states to the RibbonBarElement.ApplicationButtonElement.ActionButton.ButtonFillElement.BackgroundShape property. Thus, the image shape is blue for the normal state and orange for the MouseOver state. You can open the theme in Visual Style Builder and customize the theme according to your requirement with the desired colored shapes for the different states.
Alternatively, you can handle the MouseEnter and MouseLeave events of the RibbonBarElement.ApplicationButtonElement.ActionButton and set its BackgroundShape property using a custom image that you have. The result is illustrated in the attached gif file.