Appologies if I've misread/misunderstood, but plug colour has always been a guideline I've used to dictate engine ignition / mixture positions for roughing. Too white - lean, black as fudge - rich, coke colour - good enough to get you going.. http://www.bikerenews.com/Stories_Archives/Spark_Plugs/Spark_Plugs.html
I know it doesn't give a full dictation through the rev band, but it gives you a rough idea.