GND = Graduated Neutral Density. Which means they are neutral gray, and will only reduce the amount of light where it is darker. Nothing to do with color.
The color ones you mentioned are called Graduated Color filters. These are no longer necessary IMHO if you are shooting digital. you can add graduated color in PP easily. They are only used in film photography when are are trying to attain a certain specific effect.
If you want to do exposure balancing, GND are the way to go. But which one to get, really depends on your needs, and how you shoot. You need to do your calculations when at the scene and select the strength of the GND on the spot.
That said, I think the most useful ones are 2, 3 and 4 stops. But 4 stop GND are not easy to come by. I only pack a 2 stop and a 3 stop. My 1 stop GND is quite unused (bought it as a set of 3) and I leave it at home most of the time.