My sparkplug (2-stroke) is half black/half white. Can be related to what..?

The plugs should be clean and dry all the time, if they're not it means that for one reason or another, the cylinder isn't firing when it needs to or isn't firing all the time. It could be that you are running to rich a mixture, which will delay ignition and cause an incomplete fuel burn. This would also run the risk of fouling the spark plug, which I'm assuming is the reason you pulled it out. You need to re-jet the carburetter to give it less fuel.