well, after 100k miles i expect a bit of discoloration. Heck, the threads have more "discoloration" than the porcelain.
let see, ave 2,500rpm, v6, 100k miles, 40mph, the moon's phase is....that's about 400 million sparks?....on one plug.
the center electrode "looked" pretty much identical to the ones I put in Saturday, I couldn't get the calipers down in there...
I can't explain it, but it's why i replaced them with another set just like 'em.... I only hope the rest of the car holds up another 100k.