I scanned a Wehrpass tonight. It was issued in 1940, to a soldier who served from 1941 until his death in 1944. Here are all of the ink stamps used in the pages of this Wehrpass.
This Wehrpass has entries from the soldier’s recruiting headquarters, several military units, various officers and a doctor. There are entries made at various places and times. You will note that all of the stamp ink used for these is purple or blue, in various shades, with the exception of two stamps (stamped at the same time) for which red ink is used. WWII German military documents used stamp ink of varying colors, including black and green. While some specific types of stamps may have been intended for use with red ink, it seems that in general, the clerks used whatever was available, which most of the time must have been purple or blue ink.
I don’t believe that there was any specific purple or blue shade that was intended to be used. Ink from the same stamp pad may appear lighter or darker at different times depending on how wet or dry it is and how hard the stamp is pressed into the pad or the paper. The material the stamp itself is made from (rubber or metal) and whether or not the stamp was cleaned since the last time it was used, may also play a role. It’s also likely that the shade of some of the ink used for these stamps decades ago has shifted over time.
Here are some of the modern stamp ink pads I use for reproduction paperwork that I work on at home. I bought these mostly at art and crafts stores. Most of these manufacturers make stamps in other blue and purple colors and shades; I show here one pad from each manufacturer only. Any time I am in a store like that, I will have a look at the stamp pads. If there are inexpensive stamp pads that I don’t have, I will buy all the blue and purple shades available, and I use them interchangeably. Some of this ink is dye ink and some is pigment ink. I don’t know what was used in WWII. I don’t really notice a difference, in the appearance on the page. I don’t have a favorite, here. I have never found a modern made blue or purple stamp pad that I felt was not usable for WWII document recreations.
In addition to these, I have a lot of unlabeled pads I have amassed over the years, as well as vintage pads and even some vintage refill ink. And of course I have a bunch of different red and black stamp pads, and some other colors too.