property ToolTip.Transparency as Long
Specifies the tooltip's transparency.

Long A long expression that specifies the transparency of the tooltip, 0 means fully opaque, 50 means semi-transparent while 100 means completely transparent.
By default, the Transparency property is 0. The Transparency property has no effect if it is 0. Use the Transparency property to display semi-transparent tooltips. The TransparencyInside property specifies the transparency for inside tool tip if it is displayed. The Transparency property gets or sets a value indicating the percentage of transparency of the tooltip. The transparency is a value from 0 to 100; 0 indicating fully opaque and 100 indicating a completely transparent tooltip.

The following screen shot shows a semi-transparent tooltip ( Transparency = 50 ):

The following screen shot shows an opaque tooltip ( Transparency = 0, by default ):