First, index your throttle twist grip with a marker pen on the body and a white mark on the grip so you'll know what tuning range you're in.
Next, when you encounter this problem, close the air lever to enrichen the mixture, observing the throttle range. If the power increases, enrichen the appropriate carb setting. If the power falls off, you'll have to stop the bike, lean out the carb setting, and try it again. (My experience with reading plugs is poor; one has to kill the engine at the throttle setting of interest, pull the clutch, coast to a stop, pull the plugs then and there, read them, adjust the carb, install fresh plugs and try it again). For me, it's the performance difference that's meaningful, not plug color per se.
Ignition timing is important too, and you can fiddle with that via the 'bar lever, but only into the retard region. Opening the points gap one flat at a time (maximum two) will advance the timing slightly and is a means to see if advancing the timing is needed.
In any case, do one adjustment (and only one) at a time, or you'll chase the problem for ever.
David