property RadialMenu.ItemsBackAlpha as Byte
Specifies the value of alpha / opacity channel to show the items portion of the radial menu.

TypeDescription
Byte A BYTE expression that specifies the value of alpha / opacity channel to show the items portion of the radial menu. 255 indicates opaque, 128 semi-transparent, and 0 fully transparent.

By default, the The ItemsBackAlpha property is 255. The ItemsBackColor / ItemsBackAlpha property Specifies the color to show the items portion of the radial menu. The BackColor property specifies the control's background color. The ParentBackColor / ParentBackAlpha property specifies the color / transparency to show the parent portion of the radial menu. The ParentSize property specifies the size to display the parent zone.  The SubItemsBackColor / SubItemsBackAlpha property specifies the color to show the sub- items zone of the radial menu. The SubItemsSize property specifies the size to display the sub-items zone. 

The following screen shot, shows the portions/parts/zones of the radial menu: