Cokin graduated ND filters are just the ticket for landscapes where the sky is particularly bright -and thus susceptible to being over-exposed - or when you want a darker, moodier sky.
The filter retains normal colour balance across the visible spectrum, with a progressive graduation from the middle to the top of the filter.
For those familiar with the Cokin line, the P121M filter has the same colour as the 121 filter, but a lower tone.