CF.Lumen is a free app for Android allowing color temperature to be modified to achieve a reduction in blue light output.  It also features a darken filter to darken the image displayed resulting in an even lower minimum brightness than is possible without the app. The darken filter becomes quite important for devices that flicker at lower screen brightness range, as it allows the screen brightness to remain above the flicker threshold but by darkening the image displayed lowers the total light output.

While the user interface is cumbersome, having both color temperature and darken filters in one app is most helpful.