There is no integrated nightlight function in Zorin Lite and Linux Mint XFCE. I have therefore installed the XSCT software to adjust the colour temperature of the screen.
With the command ‘xsct 3500’ you get warmer colours with a reduced blue light component.
What is the normal colour temperature setting for laptop screens during the day? I have found various specifications such as 5000 Kelvin and 6500 Kelvin. Now I don't know which number I should set to get a normal screen again.
The consensus appears to be 6500 going by the D65 standard.
D65 closely approximates Daytime outdoor ambient lighting.
D50 closely approximates Daytime indoor artificial lighting.
This probably mostly is determined by your preferences. 
Summary
Der Konsens scheint 6500 zu sein, die dem D65-Standard entsprechen.
D65 kommt der Umgebungsbeleuchtung tagsüber im Freien sehr nahe.
D50 kommt der künstlichen Beleuchtung von Innenräumen tagsüber sehr nahe.
Dies wird wahrscheinlich hauptsächlich von Ihren Vorlieben bestimmt.
Thanks for the advice! Is there any way to save the settings so that the screen uses the same settings the next time I reboot? XSCT doesn't seem to start automatically when you boot the computer, or it probably starts with the value xfce 0. The nightlight is gone every time.
The usual simplest way is to export them in your ~.profile
This way, that command is run upon every log in.
How can I do this? I'd prefer setting it to use the last given command or set it to xsct = 3500.
What is the actual command that you are running?
xsct = 3500
At the end of the file ~.profile
add the line
export xsct=3500
Save the file - reboot and test.
Yes, that file is the one.
xsct 3500
It the very end after that last fi should work.
Arrow key down to the end of that fi, hit enter once, then ctl-v.
It doesn't work. There is a warning after reboot: '...Export:"3500" ist kein gültiger Bezeichner'
Perhaps in English "3500 is not a valid identifier"
which xsct
This should show the Path to xsct
Then use that path to create the command.
Usually it is in opt or /usr/bin
If opt, it would look like
/opt/xsct 3500
If /usr/bin
/usr/bin/xsct 3500
I tried it by adding "export /usr/bin/xsct 3500" to the .profile-file. Did you mean it this way or without "export"?
After reboot the warning ...no valid identifier...appeared again.
Then I tried typing the commands in this window (automatic startup-programs):
Does this have the same effect as adding it to the .profile-file?
It didn´t help.
I found this description in a debian forum, perhaps the command at the bottom of the page could be customized for using xsct (without using redshift, only xsct)? But I personally don´t understand the command and can´t assess it.
I tried another way of managing the colourtemperature of my screen by adding two starters to my taskbar. When I click on the red button the screen changes to 3500 Kelvin and when I click on the blue botton (next to bluetooth-icon) the screen changes to 6500 Kelvin.
I´d like to see which setting is used at the moment. The icons aren´t marked unfortunately.
I didn´t find a way to use a simple red or blue button as icon. How can I choose any picture as icon that is not a part of the systems apps icons?
Did you run
which xsct
first?
They suggest a delay to allow proper initialization. The sleep command creates the delay, the next value is the time amount for the delay and && adds a new command to the string conditional on the first command successfully running.
The plugin for that item on the panel would need a checkbox function, that denotes it is switched on or off.
Normally, I would answer this easily but since Gnome 4+, Gnome has made such customizing harder and harder to do. They really do not want users controlling their own desktops.
Yes, I ran "which xsct" first.
You are on X11, not Wayland, right?
That should have worked, though for command, I would have put
/usr/bin/xsct 3500
I do not see any way Mint could be different in this regard. And /usr/bin/xsct 3500 works successfully when run from terminal?
The command "/usr/bin/xsct 3500" runs well in the terminal.
I try it as automatical start-up-program.
Edit: It doesn't work.
This is a strange one... Something is interferring and it clearly is not xsct.
Can you please check the startup entry you made? It should be in ~/.local/share/applications as a .desktop file.
Check these values:
[Desktop Entry]
Type=Application
Exec=/usr/bin/xsct 3500
Hidden=false
NoDisplay=false
X-GNOME-Autostart-enabled=true