I was rereading the series and got to the side stories of part 4 volume 9, where a side story told from the point of view of Kamil mentions that all the paint from the buildings got washed off during the city wide waschen. My theory is that waschen works on anything small enough to be considered a particle or substance, as waschen was used during another side story with the Karstedt and Giebe Haldenzel to clean the bowls they ate their knight rations out of. The main story details briefly (if I recall correctly) that the knight rations are dehydrated soup stock that you dissolve in water before eating.
We also know waschen works on particles small enough to be left by an object simply being placed on a surface, as it is apparently standard practice to clean off the mana measuring tool used to determine the purity and types of mana in an ingredient when brewing.
My understanding of dyeing cloth is that small bits of the dye get caught up in the cloth, and that is what actually gives off the colour, which is why colours can fade after being washed many times as the particles get pulled out of the cloth. This should theoretically mean that waschen destroys the dye caught in the cloth, but as people have used waschen on themselves and others, and not once does it mention that their clothes lose colour, it seems right to assume that the dye stays.
I typically have a poor memory, so if anyone remembers some scene that describes waschen working in specific ways, or my rationality is off somewhere, I would like to hear it.
P.S. I am not exactly sure how I should be spoiler taging things, as this post covers instances from multiple volumes, and I very rarely post things on reddit. I have read the whole series, save for the side story collections, but I don't care about spoilers regardless, and I have the post itself tagged spoiler to hopefully prevent anime watchers or first time readers from seeing anything without consent. Sorry if I have done something incorrectly.