i read a statement before that the selection of spark plug departs on the driver's style / habbit
the used spark plug will reflects the true condition,
the darker colour of electrode plug, the hotter (heat range) plug is required
The whitten colour of electrode plug, the cooler plug is required
the ideally colour is brown
the smaller number of heat range, the hotter plug it is
the larger number of heat range, the cooler plug it is