Basically, the aircraft has a limited icing clearance. As a pilot you should only fly in icing conditions if there is a positive temperature band which you can descend into to shed any ice if the icing buildup starts to exceed your clearance.
That doesn't mean you can't fly in temperatures below 0. You can - providing you can avoid any visible mositure which can be difficult at night. If the aircraft starts to collect ice and surface temperatures are below zero then you can't get rid of it. So you are then in the position of losing performance and eventually descending if the build up of ice continues.
I once heard a theory about 'supercooling the aircraft' to below -5 after which icing doesn't occur. I can't remember exectly how it worked or if it worked, so perhaps the panel can enlighten me on that one!