Most of your ebooks will continue to rely heavily of black ink for their text. Color will by comparison be lightly used. To evaluate a color ebook e-ink panel I suggest you will want to see a full page of black 14pt text and a full page color image of a human face from top of eyebrows to the bottom of the chin.
If they won't show them, the panel isn't real yet (e.g. I'm not impressed by the flag example shown)