I completely agree that peak transmission is less important than the transmission curve itself. A bit offt-topic because it is not about eyepieces but whole binoculars: Last summer we rented a pair of Pentax 8x42 DCF binoculars at ATHOS star camp on La Palma island. The views could have been beautiful but I immediately was distracted by the lack of truly red stars. Everything was shifted to the blue, even Antares looked very pale.
After being back home, I found an interesting website with reviews and comparisons of a broad selection of binoculars, http://www.allbinos.com/. They had tested not exactly the same bino, but a Pentax SD 9x42 WP: https://www.allbinos...SD_9x42_WP.html. The published transmission curve and the comments were somewhat shocking - but they gave an exact explanation about why the red stars were so dim. I guess that the coatings of both binos are similar.
It is difficult to tell apart the contribution of the objectives, the prisms and the eyepieces to the transmission curve but certainly the eyepieces play a role in color rendition.
I always prefer a neutral color rendition over warm or cold hues, be it for planets or for stars.
A slanted or peaked transmission-by-wavelength curve can yield odd results.
I am not one who particularly is sensitive to "tone" in eyepieces, so I figured out a way to see the differences easily.
Most eyepieces just seem "normal" after a minute of looking.
In the daytime, I have a white building a few miles away that is large enough I can see its color quite easily with my naked eye.
It also has a red windsock on the top that is easy to spot with no optical aid. It can help reveal if red is over-prominent or subdued.
I aim the telescope at the building and insert an eyepiece. Moving quickly from eyepiece to naked eye view and back again, very small tonal biases can be detected.
The most common tones I see in eyepieces are a yellowing of the white in the building. I see this in many if not most eyepieces that use lanthanum oxide glass.
But I have also seen a bias toward the blue, and even one that was decidedly pink! I'm not exactly sure what to think when I see (and it's in many eyepieces) the white of the building
become a light grey in the eyepiece. Does this indicate a lower transmission overall? If not, what does it say about the tonal character of the eyepiece? I'm not sure.
It may simply be a darkening due to magnification, because not all the eyepieces I've tested this way have been low power eyepieces.
At any rate, unless the tonality is particularly egregious, I don't usually remark about it.
This test reveals if there is any chromatic aberration of the exit pupil as well, or simple chromatic smear in the outer field, merely by moving the white building back and forth in the field.
I used to sell binoculars, however, and the color tint variations in binoculars are much larger than those in telescope eyepieces. So Juergen's finding is not unusual.