This has been fixed in Ubuntu 20.04!
(There is now a slider in Ubuntu 20.04 and newer to manually adjust the color temperature.)
Ubuntu 18.04
By default, the color temperature for Night Light is 4000, which was way too orange for me.
You may find this too harsh. You can adjust the color from a terminal with this command:
gsettings set org.gnome.settings-daemon.plugins.color night-light-temperature '5500'
You can change it back to the default of 4000 with this command:
gsettings set org.gnome.settings-daemon.plugins.color night-light-temperature '4000'