Transparency Property [Excel 2003 VBA Language Reference]
Returns or sets the degree of transparency of the specified fill as a value from 0.0 (opaque) through 1.0 (clear). Read/write Double.
The value of this property affects the appearance of solid-colored fills and lines only; it has no effect on the appearance of patterned lines or patterned, gradient, picture, or textured fills.
This example sets the shadow of shape three on worksheet one to semitransparent red. If the shape doesn't already have a shadow, this example adds one to it.
With Worksheets(1).Shapes(3).Shadow .Visible = True .ForeColor.RGB = RGB(255, 0, 0) .Transparency = 0.5 End With