Would be cool to wear a "light panel" on your sleeve.

Outdoors you see the e-ink fine. Indoors you look down at your illuminated sleeve to see the e-ink in your glasses.

You can't backlight an eink display. They are opaque, by design. Thats why they make such great ebook displays, so you can take them outside and still read them. Eink displays require .. i guess 'forelight'. Light that reflects off the display front.

I didn't know that, I just wonder how the kindle with light works though. Is that light coming from the front too?

If you look closely at a backlit kindle screen, you can see that the e-ink pixels are a decent distance behind the surface. It’s more noticeable if there are scratches or dust on the screen and you rotate the display. The surface reflects the light from the sides back to the e-ink.

frontlit

IIRC, side illuminated microscopic dots on a protective cover. Light only escapes through the dots, illuminating the panel, without shining into the user.