Allow me to suggest that, if you haven't looked at handheld computers or smartphones, you might do that. These devices will read e-books... you can download multiple e-book readers, so you can read books in many different e-book formats... and you get color screens.
The issue with handhelds and smartphones tends to be size... not everyone can get used to reading on a 2x3" screen. Also, some are uncomfortable with long-time viewing on a handheld screen (though I've never had a problem, plenty of others in this site don't like it).
On the upside, handheld PCs and smartphones have a lot of other uses, and entertainments, available to them, and you might like that. And they can cost significantly less than an e-book reader.
Do the same kind of comparisons with handhelds that NatCh suggests for the other devices/services.