When you spray nitrous it causes the internale temperature to rise. This is due to the added amount of O2 into the engine. If you run this on your stock spark plugs, it will essentially turn them into a glow plug. This allows for the possibility of pre-ignition since the plug insulator is very hot. Pre-ignition is bad, VERY BAD. It can cause all sorts of damage to an engine.
A colder plug has a shorter insulator on it which allows the plug to run cooler. The insulator is the part that gets the hottest on spark plugs.
Check out
http://www.ngksparkplugs.com/techinf...overviewp2.asp . It explains it in depth.