Bluelight Filter is a free (and ad-free) Android app which aims to reduce the effects of the blue light emitted by smartphones and tablets.
It's claimed, with some support, that this can reduce eye strain, help you sleep better, and generally make the screen easier to read.
The app comes with multiple filters, each with their own tint. You can select whatever you want or add a new filter of your own.
Full control of opacity levels means the program also doubles as a screen dimmer.
An optional scheduler can turn the effect on and off at set times, perhaps dimming and softening the screen in the evening.
Generally this works well and it shouldn't get in your way, but if you do have any issues then the filter can easily be toggled on and off from the status bar.
An excellent screen filter with plenty of features, which unusually is also very small (<100KB), ad-free, and doesn't require any permissions.