I started this thread (pun intended) so obviously I am not an authority but here is what I understand. Yes, NCP means "no color perserver" needed for the thread to look like the same color off the spool as after you put epoxy on it. The downsides are less colors and less vibrancy in the actual color. The are a bit flat and very opaque.
On the rod I just finished I used ProWrap brand nylon for the tan and green and Fish Hawk NCP for the trim bands. If you look back at that rod build thread you can see the difference in the vibrancy of the colors before epoxy...and after. Nylon thread without color preserver will always have an amount of transparency, so the color will change depending on the color of the blank under it (or any underwrap). Again on my XP I used tan, which looked almost non-metallic gold, but after the finish the green of the blank made it much darker and the guide feet are clearly visible under the transparent thread. If I'd used color preserver, I imagine the tan would have stayed much closer to the unfinished look.
"Raw" tan nylon and lt. gray NCP:
As for the characteristics of the threads and my limited experience with the NCP, I much prefer nylon. The Fish Hawk NCP tended to fray and unravel a bit when making small trim band. Nylon stays in its nice little "rope" no matter.
Other members here like Diver Dan and Swirlchaser, just to name a couple, are incredibly experienced thread masters, if you will. They do amazing stuff and will hopefully give you better and more info.