Each picture is a thumbnail, meaning you can click it for a full-size view. T he Spark plug to the left is what a normal plug should look like. Grayish-tan to white in color indicates the plug is operating at the proper heat range as well as correct jetting and the cylinder is running healthy. That vertical color band on the ceramic shows you where the plug is indexed. Meaning that band is aimed slightly at the exhaust valve.
Optimum location places the band HERE .
If the band is anywhere but here, it means that when the plug is not at it's optimum location. There is still discussion as to whether indexing a plug is worthwhile, but on may applications looking for that last horsepower, it doesn't hurt. This is caused by foreign objects in the combustion chamber or an improper plug reach where it contacts the piston. Even a piece of carbon can do this. To solve this, make sure you have the correct length tip spark plug as well as removing any foreign materials in the combustion chamber.