Use Not IsNull(Fields!W_O_Count.Value)
So:
=IIF(Fields!approved.Value = "N" & Not IsNull(Fields!W_O_Count.Value)), "Red", "Transparent")
Use Not IsNull(Fields!W_O_Count.Value)
So:
=IIF(Fields!approved.Value = "N" & Not IsNull(Fields!W_O_Count.Value)), "Red", "Transparent")