As per our current level of knowledge, there is no thorough examination of the colors for the red background lines present on the Zurich stamps. Gnägi is writing in his book about the Zurich cantonal stamps on page 24, that the color may vary from light pink to crimson and brown red, dark brown and gray. We may thus assume that he knew a variety of shades. In a letter from the end of the 1970s, Max Hertsch wrote me that he discovered a piece showing brown background lines. Both authors however have not produced any studies about the topic to this date, we would like to elaborate our findings in a little more detail. The recent purchase of two fitting Zurich 6 stamps came in handy.
2 W, type II, 92th stamp: using binoculars, traces of red color can be found among the gray color. A mere magnifier leaves the impression of brown background print, as a secondary color, one might say. The red traces appear towards the edges of the gray lines, appearing like a drop shadow. A conclusion might be that the lithographic stone was not properly cleaned for the red background print and gray color was simply applied on top. The (dried out) red color traces were then printed as well, albeit significantly less visible than the gray base color.
2 W, type IV, 94th stamp: the stamp shows clear gray background lines. There are hardly any red traces. This could be dubbed a “later version” of the type II/92 described above. The originally distinct traces of red color vanished as a result of continuous printing or re-application of gray color.
We now know that differences in the background lines are recognized since Reuterskjöld (about 100 years ago). An explanation most common would be the fading of original colors caused by light, rudimentary cleaning or even chemical treatment. This phenomenon can also be observed on the 3rd edition of Zurich stamps, which originally did not have any background lines. For decades, these stamps were also referred to and certified as faded.
In the end, we do not want to rule out this opinion entirely, even if it seems rather improbable from our point of view. Otherwise, the question would arise why only the Zurich stamps fade in strong sunlight. These later prints were printed on low quality porous paper and gray color was used instead of black color. Thus, they seemed faded from the beginning. Early stamps bearing horizontal lines can hardly be found without the background print.
175 years after the first Zurich stamps were issued, interested collectors, dealers or certifiers should tend towards that topic. The classic Swiss stamps pose one of the most interesting philatelic areas. The fact that new insights can be gained even 175 years later should be the best proof.