Tim,
The grey is referred to as "jacket" and this wire is legal on FRC robots where color coding is required. There is a weight disadvantage though. The jacket is often filled with clay and can be hydroscopic meaning it will absorb moisture from the air. While this amount is small, added to the additional weight of the jacket might pick up more weight than you want. The zip cord available from places like MCM Electronics works as well, doesn't carry any extra weight and is easier to prepare. Check the varieties and quantity here...
http://electronics.mcmelectronics.co...p+cord&x=0&y=0
I currently use the jacketed variety for feeding speakers in the studios here.