Herpa Premiums models originated from the Lufthansa Modell Edition. These were limited edition models produced by Herpa in the late '80's for Lufthansa. Lufthansa wanted these models to be the absolute most detailed and accurate models ever in the 1:200 scale (and they were).
So what does this have to do with the sandwich effect?
When Herpa produced these models, they wanted a perfect, crisp cheatline for the Lufthansa livery of the time, so they designed the fuselage mold in 3 pieces. The top would be molded in white, the middle section with the windows would be blue, and the bottom molded in gray. By combining these three pieces, they achieved a crisp cheatline. However, when Herpa released the Premium Series which included liveries that were all white, they used the molds from the Lufthansa Modell Edition. The result was the "sandwich effect" of 3 pieces molded in the same color coming together. Molded as one piece, the models would look much better. But, from a distance, it is hardly noticeable. The 747 is molded in 2 pieces, so there isn't really a problem with the sandwich effect there.
All that said, Premiums are still beautiful, high quality models, and most people can look past the sandwich effect. I hope Herpa will one day develop new molds for use on liveries with a solid color fuselage, but I understand the molds are very
expensive to develop.
Take a look at these pictures, they show the sandwich effect pretty clearly.
Well, I hope I answered your question thoroughly
Let me know if you want any more info or pics of different Premiums.