OH, I missed where this is a 6 pack.
I would have to side with Yellow Rose then on it being a tuning issue (exept all the plugs apparently look the same.)
If it was a tuning issue you would think the four centers would be whiter than the outers.
So you don't think it's possible that the 4 center cylinders having their exhaust ports right beside each other compounding each others heat might have anything to do with it?
The plugs apparently say the cylinders are burning at about the same temps.