Schwinn ads and catalog pictures are many times not accurate. They are a copy that was done months even years before it was used. Dec 63 art work was probably done the summer before and maybe just a picture they had on file. None of it would have been done on a computer but would actually have been hand painted. It was done to sell bikes not for future collectors to use as a reference. Roger
Looks like a simple error on the part of the graphic artist or editor, they mistakenly used an image of a 1962 model in the Dec. '63 ad. That same ad also has a typo in the Varsity wheel size, perhaps it was layed out on a Monday. It is interesting that those same errors were not in the Oct. '63 ad. I suppose it is possible that the Christmas ad had been worked on first.
It is actually Kool Lemon, and black decals were standard for that color. I've seen that ad many times before and many copies or photos have the colors way off, either due to age, camera settings, etc. I believe the Jacket worn by the guy in the middle was also yellow. Here is another example where it looks a little more yellow: