Normally all of the plugs will burn a similar color which should be a medium brown.
In your case, where one is way out of whack, I'd probably start by replacing just that plug, or by cleaning and swapping it with one from another cylinder. Another possibility could be a deteriorated spark plug wire. If you still have the old set you could borrow one from it. One item that is often overlooked is the spark arrestor, which will gradually become gunked up and reduce the system's air flow. Because of the PCV valve's job of removing oily air (blow-by) from the crankcase, they are also prone to becoming dirty. If it sticks in the closed position, it will definitely richen the mixture.
Tom
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