Thanks Fledchen for your helpful post.
My 9 year old son uses coloured overlays with my Sony Reader and we find it works quite well. I have used Book Designer to enlarge the font for him as well and would recommend it.
When I first joined the forum I received some useful information from Elsi and Andybaby about overlays, reading difficulties, and e-ink --
https://www.mobileread.com/forums/showthread.php?t=35986