Here is a brief description of "hot" and "cold".

I run NGK's in both my outboards as recommended by the manufacturer.

"Spark plugs....We can make this easy or we can make it hard. Let's start with easy. After running for a while normally, stop the bike and take the spark plugs out. They should look a light chocolate brown. If, black and sooty the engine is running too rich in the fuel departement or too cold of spark plug. If black and shiny they are oil fouled. If they are whitish, you are running too lean a fuel mixture and/or wrong ignition timing or too hot of spark plug. When you see this, your engine is about to blow up!!!

Now the harder stuff. A spark plug does not make your engine run hotter or colder. The terms hot or cold refer to the temperature of the tip of the spark plug itself. The plug must keep a temperature of, between 752 and 1652 degrees Fahrenheit. If it gets too hot it can fire off the air/fuel mixture at the wrong time. This can cause detonation and/or pinging which will put a hole in your piston... not good. If it gets too cold it will not burn off the carbon and fuel deposits on it's tip and the plug will foul out (stop working) not good either.

The heat range of a spark plug is controled by the length of the center electrode. A longer one is hotter because it takes longer for the heat to flow through it to the cylinder head. As you might think the cooler plug has a short center electrode."

Edit: One last thing. A hotter plug generally makes for a better idling engine.