"Instrument Flight Rules. A set of rules governing the conduct of flight under
instrument meteorological conditions." VIA: http://www.fly.faa.gov/Products/Glossary_of_Terms/glossary_of_terms.pdf
Just because its dark does not mean you are flying IFR. You can still see city lights if you in a populated area or stars or the moon. But when you are flying in fog or snow and you cant see a mile if front of the windscreen, and you have to use your instruments and hud to give you attitude and altitude indications, then your considered in IFR conditions.
As for the question, what Riboyster said is what works for me. I turn my gain all the way down till its blurry and then add 1 increase key press, and turn the brightness all the way down on my MFCD's and HUD color to night and also lower the intensity of that and I usually don't touch it for the rest of the flight time. I found that I can read and distinguish everything displayed on the TAD and TGP as long as the FLIR is in Black Hot Mode. White is just too bright there is nothing you can do about it, but look at it with your NVG's off.
Hope this helps.