Early "Cold Start Throttle", effects on spark plugs??

[H.G.P.]

Well I pulled the spark plugs tonight and most were of decent appearance brown to grayish electrodes as Haynes p. 83. I would judge them as "normal leaning toward carbon rich". (No electrodes had carbon though). This was after three months of hard driving, short distances (1-10 miles per trip) daily driving.

My question is, the carbon "soot" that appears on some of the plug threads, can this be caused by the inexact graduation due to cold start throttle use? And does this effect spark plug life?

Thanks