You have to be careful with filter descriptions, as W.Smith says, the Grey Grad doesn't specifically say it's a ND filter, but it could easily be an omission. If it's ND, then it will have no effect on the colour transmitted by the filter, just reduce the amount of light being transmitted.
If it's not ND then the darker area could have an effect on the colour. Cokin make gazillions of grad filters for all sorts of weird effects, so I'd be a bit careful. Give WE a ring -they're usually pretty helpful with any queries.