I have played with the tiny 1.5" color eink displays from Waveshare and I decided to go big so I got a 4.26" grayscale capable non-color one with fast refresh for an outdoor handheld mapping GPS and e-compass project. I was pleased to see, as I expected, that the display was very readable in bright sunlight. Imagine my horror though, when I refreshed it in bright sunlight and suddenly it was very low-contrast and faded! I certainly hadn't seen this behaviour with the small one I tried first.
Then I noticed, almost by accident, that if I was shading the spot on the display where the ribbon cable came in, the faded display effect DID NOT APPEAR during a refresh in sunlight. I noticed that there was a translucent bar there, and I suspect that the controller chip is under it. Remembering somewhere that the display uses an on-board temperature sensor, I wondered if the bright light was affecting it. (reminiscent of that bug in the raspberry pi 2 where photographing it using a flash made it turn itself off....)
So that's my solution. Cover that part of the display to stop the sunlight from hitting the controller chip or sensor or whatever. Doesn't have to block all light, in case that matters, just direct sunlight.