The reason you should use colder spark plugs is so that the colder spark plugs can dissipate the heat more quickly and prevent the tip of the spark plug from overheating and possibly causing damage to the engine, pre ignition or detonation.
Colder spark plugs are used when the engine is running hotter than it normally would like in turbocharged engines, supercharged vehicles or high performance engines or when any modifications have been made which can increase the temperature of the engine.
However using a spark plug that is too hot can result in overheating and cause electrodes to wear quickly and lead to a reduction in fuel economy and pre ignition.
A hotter or colder spark plug means and refers to the thickness and the length of the spark plug's insulator nose.
A hotter spark plug will have a longer insulator nose.
And a colder spark plug will have a shorter insulator nose.
Hotter spark plugs have a longer insulator nose which means that the heat is transferred more slowly to the engine.
The heat being transferred more slowly to the engine means that it keeps the firing end of the spark plug hotter and burns off carbon deposits.
Colder spark plugs have a shorter insulator nose and means that the heat is transferred more quickly to the engine.
The heating being transferred more quickly to the engine with the shorter insulator nose keeps the firing end of the spark plug cooler and can cause deposits to build up.
The ideal spark plug heat range will depend on the engine and how it is used.
Modified engines or any engines with fuel delivery issues often require a hotter heat range spark plug.
However if you have too hot of a spark plug it can overheat and melt the electrodes and cause pre ignition.
Too cold of a spark plug can accumulate deposits and short the spark plug out which is called spark plug fouling.