Redshift + Xorg + KDE
If you're running KDE and Xorg, you'll likely need to use Redshift to tint your screen at night. "Redshift Control" is a KDE widget that gives you a nice GUI to configure Redshift (found by searching the "Get new widgets" interface).
I also found (in this video) what might be the command line way to install both redshift and the plasma widget:
sudo apt install redshift plasma-applet-redshift-control
Although elsewhere I've seen an additional program recommended:
sudo apt-get install plasma-widget-redshift plasma-applet-redshift-control
Configuring Redshift in Plasma
First, I'd recommend moving the widget to the task bar. Then, to configure it, simply right-click the widget and click configure. Hit "Locate" to automatically locate your position. Then for temperature 6500 for Day and 3250 for Night seems to work well.
For gtk-based desktop environments like GNOME, you're probably going to want to run
sudo apt-get install redshift redshift-gtk