Google rolled out a blue light filter feature for Google Play Books a few days ago. You can now install the latest version of GPB for iOS or Android and use Night Light, as the feature is named, but I don't think there's any hurry to do so.
I installed the new Google Play Books Android app last night, and tested it while I was trying the Fire tablet's new Blue Shade feature. And frankly, I'm not impressed.
Google's feature is so simple that it strikes me as a "me-too" addition rather than a planned feature.
Night Shade is an automatic filter that adjusts with the time of day. There's a single switch to toggle it on and off (look in the font/formatting menu) but no way to adjust the color, strength, or what have you.
You can just turn it on or off, and that's it.
When you do turn it on, you won't see anything for most of the day, but as the evening progresses into night your screen will look more and more like the screenshot at right.
Both screenshots were taken with a white background; I didn't think to try the other backgrounds, sorry.
That's not much of a blue light filter, so if you're reading on Android you're really better off using a different app.
Android has quite a few apps in this department, and the best one by far would be Twilight. It's the only blue light filter that has been recommended to me, and now that I have installed it I can see why (it's so good I'm about to upgrade to the paid app).
Twilight lets you enable or disable the filter manually, or set your own schedule, or set it to automatically change with the time of day. You can also adjust the color temperature and intensity.
It's a great app, and well worth the $3.
But if you're reading on iOS, your options are much more limited. There are a double handful of web browsers with integrated blue light filters, but ebook apps are still thin on the ground and Apple won't let anyone modify system setting like screen color (so a system-wide solution like Twilight or F.lux is out).
As I explained in my post on reading at night, many ebook apps for iOS, including Kindle, iBooks, Aldiko, Google play books, and Kobo, have a night mode where the text and background are inverted (white text on a black background).
Some readers swear by that mode, but if you really want a blue light filter on on your iPhone or iPad, your best bet would be to buy a pair of glasses that have been tinted to block blue light, use the GPB app, or wait for Apple to add this as a core feature of iOS.
I have no evidence to back it up, but I am expecting that to happen with the next major update if only because Apple will want to copy Amazon (Google will have a similar motivation with Android).