Ahh yes my friends, I have the answer for you...
In order to get the night stalker scope white hot mode (the best and most clearest) you must do the following.
First switch weapon in its secondary view mode, by pressing Ctrl+Right mouse click, then do it again, so that you are back into main scope optics mode, then simply press 'N' and white hot mode should come up
If not, keep on switching between the two optics view mode, and press 'N', it will eventually work.