I believe of the bottle caps, number 11 is the hardest to find. It is a very neon yellow that was originally sold as part of the Elek-Trick series on a yellow base. I remember seeing a picture of one that came with a Princess base and cap somewhere on here, but I cannot find it at the moment. In general, maybe an original Century 101. Red lava in fluorescent green liquid. There are a couple lucky members who have some examples. It's kind of hard to say which color of them all is the rarest; it depends on which era of lamps specifically you are looking at. Hope this helps!
Think the chocolate to the Aztec and there is a raspberry color that come to mind for me, not sure the number codes though. Sure the others will chime in.
Ok thank you for the information I was just curious manly because I got a bunch of new lamps from the 90s
I vote Yellow Liquid and Lime Lava is pretty hard to find