In the Grimms Kinder und Hausmärchen text Vom klugen Schneiderlein, 1815 #28 there is a line:
Da sprach die Prinzessin: »ich habe zweierlei Haar auf dem Kopf, von was für Farben ist das?« »Wenn’s weiter nichts ist, sagte der erste, es wird schwarz und weiß seyn, wie Kümmel und Salz.«
In 1850 the line is changed as follows:
Da sprach die Prinzessin »ich habe zweierlei Haar auf dem Kopf, von was für Farben ist das?« »Wenns weiter nichts ist,« sagte der erste, »es wird schwarz und weiß sein, wie Tuch, das man Kümmel und Salz nennt.«
The questions are:
are there any literary references to a type of cloth that is called “Kümmel und Salz”?
are there any images of cloth that is called “Kümmel und Salz”?
is “Kümmel und Salz” an idiom (eine Redensart)?
That the words are supposed to express the idea of something "black and white" can be seen from the context of the text.
A search at the DTa gives 161 examples of the phrase “Pfeffer und Salz” (pepper and salt), but it only give 10 examples of the phrase “Kümmel und salz” (caraway and salt). Of those 10 examples, 6 are from this KHM text. The other 4 are related to cooking. The modern word for black and white fabric is “Pepitastoff” as I understand it. That the phrase was might have been somewhat unclear to readers in the past can also be seen by the change in the 1850 edition where the words “wie Tuch, das man” (as the cloth, that one) was inserted before the words “Kummel und Salz” so that the phrase then read: “as the cloth, that one called Kümmel and salt.” The phrase is related to a type of cloth and it seems to be unique to the Grimms and the KHM. I have found no other references for the words “Kümmel und Salz” related to a type cloth. Also, the phrase seems to have been important enough to Wilhelm that he never changed the word “Kümmel” to “Pfeffer” or some other type of black seed. There is an entry in the 1862 book “Das Deutsche Gaunerthum,” Bd. 4. Leipzig, by Friedrich Christian Benedikt, where there are references on pg 563 and 594 to “Kümmel and Salz” being rogues language for “[black] powder and [shooting] lead.” This is not likely to be the meaning in this text. I can’t find anything on this cloth that Wilhelm Grimm describes.