A value of 0 represents a transparent brush, whereas a value of 255 represents a completely opaque brush (items behind the filled area are not visible). Values between 0 and 255 represent varying degrees of translucency.
Although it is possible to assign a color with an alpha component to Fill, developers should assign such opacity values to this property instead.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Server 2012, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2