I think a gray or whitish coating can be considered normal. Even light brown. I am wondering about the difficulty you had in removing the plugs. Made me wonder if you might have gotten a little off-axis when you were installing them, and cracked one or more of the plugs. I always use anti-seize compound on the male threads of plugs and I use a thread chaser (K-T sells them for a few bucks) to make sure the threads in the head are clean. You should be able to turn them in by hand using a plug socket and extension and do the final tightening with a handle.
Sometimes, when a problem appears right after a plug change, it is because the wire was damaged when it was removed from the plug. I like to use silicone grease on the metal clip and on the silicone boot, so the boots come off without harm in the future.
This is way more important with aluminum heads, but it can help with our cast iron heads, too.