There are many reasons why you can't use plugs for the type of fine tuning that usualy raises this issue. Racers have specific drills for checking plug color, none of which applies easily to a street motor. Let's say you ride for 500 miles, then pull the plugs. They're light tan. What does that tell you about each of the metering circuits in the carbs? What does it tell you about the ability of the carbs to produce the correct A/F ratio under various conditions, including intermittant acceleration and idle? What does it tell you about the color of the plugs just before you decelerated and shut off the motor? The answer is "nothing."
