The wallpaper is not defective yes the dye's used are not colorfast, if they get wet they bleed. That is NOT the case here. That is exactly how it's supposed to look I have hung this paper many times it is as they say the nature of the material. The problem here is not the installers or even the manufacturers, it is the designer. She failed to educate you on the "look" of this type of grasscloth (grasscloth varies in appearance depending on type) and then even worse she really failed when she threw the installer under the bus! This is typical of an inexperienced or unqualified, uncredentialed designer. The photo below is one of MANY installs I've done with this type of grasscloth.

Here's the same material being hung prior to double cutting seam, notice the shading is caused more as an effect of lighting, below that we shaded (matched) sheets prior to being hung. Final photo is of the finished install. Other colors of grasscloth will do this too. Depends mostly on fiber and weave. This is not the same effect as shading, light/dark or paneling. Often it has to be reverse hung to minimize paneling. Clients are always informed prior to install