What causes LG screen colors to change when I install a Mac OS update?
I've the same issue.. Working on a 15 inch macbook (2018) and just update to macOS Catalina 10.15.4. Macbook itself working fine but my LG 27UK850 monitor isn't showing matching colours anymore.. Tried a restart of my macbook but that didn't help. Monitor to factory settings isn't fixing anything as well.. Macbook colour repair shows me that I've around 30 bad colour profiles. Don't know if this is what has to be fixed but i can't repair it unfortunately..

If your color changes as the sun comes up and down, check for Night Shift. 

I've found how to fix it. 


Go to System Preferences 
Go to Displays 
Turn off 'High Dynamic Range' This fixed it for me. Hopefully for you as well =)


If this doesn’t fix it, consider performing a full calibration for both machines and check other settings relating to color. 


https://support.apple.com/guide/mac-help/calibrate-your-display-mchlp1109/mac
https://support.apple.com/guide/mac-help/change-your-displays-color-profile-mchlf3ddc60d/mac
https://support.apple.com/guide/mac-help/if-your-display-doesnt-look-right-mchlp2906/mac