The following VBA example fills all shape objects with the LAB gray color and displays the LAB component A value in a message box.
Sub Test()
Dim s As Shape
For Each s In ActivePage.Shapes
If s.Fill.Type = cdrUniformFill Then
s.Fill.UniformColor.LabAssign 128, 0, 0
End If
Next s
MsgBox s.Fill.UniformColor.LabComponentA
End Sub