I wouldn't want to assume that the grey PVC is suitable for water. It is probably tested to a different set of criteria for suitability, and for example might have a lower pressure rating, or might contain lead (many plastics contain small amounts of metal as stabilizing agents).
The white PVC is printed with pressure and temperature ratings; I don't recall seeing this on the grey stuff.
Grey is UV protected. I see people using it for sprinkler lines but I would not use it for potable water since we don't know what the UN protector is. The white is PVC, CPVC is more of a yellow color and smaller for a given trade size. (1/2" CPVC will fit inside 1/2" PVC) For our purposes the grey is the only one that is listed and that is the real answer. All three are very common in construction here in Florida where metals don't last very long.
Don't mix it! The manufacturing methods are such that electrical conduit would leak and the plumbing pipe would burn. If there was an electrical fault, and the pipe caught the building on fire, the installing electrician would be held responsible.
Earlydean...I agree with your conclusion (don't mix it), but not with how you got there.
According to Cantex...which makes both types at their Reno plant, using the same amchines and the same dies. here are the differences:
PVC Plumbing pipe is formulated, assuming the pipe will be buried (not exposed to sunlight), and filled with water- which helps support the pipe. Electrical PVC is rated for exposure to sunlight, and has "stiffeners" added to help keep it from sagging. Thes "stiffeners" would also, I suspect, make the pipe become brittle more quickly, so it probably wouldn't last long as plumbing pipe.
Other than the potability I doubt there are any issues with using RNC for water supplies. I see it all the time for exposed sprinkler lines, fish cleaning sinks on docks and such. It seems to work OK. The Florida sun will tear up regular white PVC. I suppose you folks have to bury water lines up there in the frozen north ;-)