There is a question amongst fans on what the caps should look like. Let me lay it out for you.
Now a little debate about the color scheme. The best picture we can get from any of the games is the one I added here. Its a Caravan bet interface from New Vegas. These caps show a brass like cap base while the Nuka Cola label has a black outline.
In game if you zoom in on a full bottle it is just a black cap.
If you zoom in on a loose cap in game its a Nuka label, no outline and on a silver base.
Since it is the best picture a lot of prop builders are going with the Caravan Cap now. But to me it doesn't really matter. Companies change designs all the time and I just put it down to that. (as part of my suspension of belief while playing.)
Now if I am going to add a cap to a bottle or mod one already on a bottle I'll probably go with the caravan cap. (the one with the brass coloring, black circle and red Nuka Cola label. On the other hand if it's just to have a collection of caps in a bag as a display or cosplay piece then I don't think it really matters. In fact variety is good. Keeps everything from looking like it came from a distributor. Heck They don't all have to be Nuka caps for that matter. Throw in a Bawls cap. Sunset Sarsaparilla caps, Gamma Gulp, your favorite micro beer cap. It's a game. If you plan on making these then your a fan of the game. (or just have a strange thing for bottle caps. Ok maybe not so strange. There is a guy who collects his belly button lint on the web ((shudder))
Anyways, sit back and enjoy the Instructable.