If you mean in the merge document itself then the following macro will do that.
Code:
Sub Macro1()
Dim oCell As Cell
Dim oFld As Field
For Each oCell In ActiveDocument.Tables(1).Range.Cells
For Each oFld In oCell.Range.Fields
If oFld.Type = wdFieldMergeField Then
If InStr(1, oFld.Code, "Product") > 0 Or _
InStr(1, oFld.Code, "Desc") > 0 Then
oCell.Shading.BackgroundPatternColor = wdColorWhite
Exit For
Else
oCell.Shading.BackgroundPatternColor = &HD9D9D9
End If
End If
Next oFld
Next oCell
lbl_Exit:
Set oCell = Nothing
Set oFld = Nothing
Exit Sub
End Sub