If you are using OpenXcom Extended you may also want to check out the options setting found in 'Options -> Advanced -> Extended -> Auto Night Vision threshold"
(https://openxcom.org/forum/index.php?action=dlattach;topic=11486.0;attach=60621)
That may be what is causing night missions to always display in a different colour as it is switching on your night vision automatically whenever the light level is below its setting.