What's the point with that quite lackluster story in Endwalker?

[EaraGrace]

You may be confusing this. He was the only one for whom they changed to BLACK (or purple-ish) specifically. That's why Meteion wanted you to change it to black too, so he could see he wasn't alone in his sorrow.

I was remarking that he’s the only one who showed any ability to effect change dynamis, limited as that manipulation may be. It’s explicitly stated that the color never changes around everyone else. It’s certainly possible that’s not the default color for Elpis flowers and they were simply all expressing the exact same emotion, but either way it’s more about how limited the ancients were to interacting with it. You can just as easily point to the way Venat, Emet, Hermes, and Hythlodaeus we’re unable to hear Meteions pleas, while the WoL could as another example.

Hermes: And those entities like the Elpis flower, that have the ability to interact with this energy, converting emotions into tangible phenomena...
Hermes: ...Are entelechies.

If a presumably soulless flower can…

The Elpis flowers respond to the emotions of others, it doesn’t effect change as it lacks a will to do so. Standing near an Elpis flower won’t make you susceptible to its influence through dynamis, but standing near Meteion? Different story.