The plug deposits look decent. But, Metalhead is on the right track, I think. If your carb runs rich at idle and WOT and then lean through midrange, the plug deposits might "average" ok.
Head pipe discoloring is usually a sign of high exhaust temp and/or oil contamination, which can etch into the chrome.
If you cleaned with WD-40, it leaves an oil film behind, clean that off with alcohol or soap & water before they get hot again.
What about carb synch? If one carb is open farther than the others, that cylinder will work harder/hotter.
In summary, if only one pipe is discoloring, there is either some surface contamination, or something about one cylinder is not operating the same as the others.
Is compression equal across the bank?
Is carburetion equal across the bank? At all throttle positions?
Is filtration equal across the bank?
Is exhaust back pressure equal across the bank?
Cheers,