Often, the option to enable the Night Light (aka Blue Light Filter,) is greyed out for no good reason, and you can't change it from within the UI. Most fixes available on search engines only work on Windows 10 obviously. All this one does is turn it on, set it to 75%, disable the schedule, etc (obviously it could be changed afterwards to whatever setting you wanted.) Save this as a .REG file:
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore]
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore\Store]
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore\Store\DefaultAccount]
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore\Store\DefaultAccount\Cloud]
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore\Store\DefaultAccount\Cloud\default$windows.data.bluelightreduction.bluelightreductionstate]
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore\Store\DefaultAccount\Cloud\default$windows.data.bluelightreduction.bluelightreductionstate\windows.data.bluelightreduction.bluelightreductionstate]
"Data"=hex:43,42,01,00,0a,00,26,b7,91,94,8d,06,00
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore\Store\DefaultAccount\Cloud\default$windows.data.bluelightreduction.settings]
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore\Store\DefaultAccount\Cloud\default$windows.data.bluelightreduction.settings\windows.data.bluelightreduction.settings]
"Data"=hex:43,42,01,00,0a,00,26,c1,99,94,8d,06,00
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore\Store\DefaultAccount\Current]
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore\Store\DefaultAccount\Current\default$windows.data.bluelightreduction.bluelightreductionstate]
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore\Store\DefaultAccount\Current\default$windows.data.bluelightreduction.bluelightreductionstate\windows.data.bluelightreduction.bluelightreductionstate]
"Data"=hex:43,42,01,00,0a,02,01,00,2a,06,b7,91,94,8d,06,2a,2b,0e,15,43,42,01,\
00,10,00,d0,0a,02,c6,14,ee,d8,f3,8b,b5,a8,f9,eb,01,00,00,00,00
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore\Store\DefaultAccount\Current\default$windows.data.bluelightreduction.settings]
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore\Store\DefaultAccount\Current\default$windows.data.bluelightreduction.settings\windows.data.bluelightreduction.settings]
"Data"=hex:43,42,01,00,0a,02,01,00,2a,06,c1,99,94,8d,06,2a,2b,0e,19,43,42,01,\
00,ca,14,0e,15,00,ca,1e,0e,07,00,cf,28,ba,27,ca,32,00,ca,3c,00,00,00,00,00
