Spark plugs for aluminum heads have a trivalent, or similar, plating on them to act as anti-seize, likely to facilitate installation by robots or robotic DIYers.
NGK likely knows more about spark plugs than any member here or any service manual for that matter. They make the plugs and have been doing so for a couple years now.
Here's what NGK says:
5 Things You Should Know About Spark Plugs - NGK Spark Plugs
5 THINGS YOU SHOULD KNOW ABOUT SPARK PLUGS
1. Anti-seize
NGK spark plugs feature trivalent plating. This silver or chrome-colored finish on the threads is designed to provide corrosion resistance against moisture and chemicals. The coating also acts as a release agent during spark plug removal. NGK spark plugs are installed at the factory dry, without lubrication or anti-seize.
Anti-seize can act as a lubricant, altering torque values up to 20 percent, increasing the risk of spark plug thread breakage and/or metal shell stretch. Thread breakage can sometimes involve removing the cylinder head for repair. Metal shell stretch changes the heat rating of the spark plug and can result in serious engine damage caused by pre-ignition. Do not use anti-seize or lubricant on NGK spark plugs. It is completely unnecessary and can be detrimental.
Other companies, like Champion, say something similar, their coating will have a different name.
The question has been asked before:
I am about to do my first changeout of the factory plugs and want to know from some of you experts what is the deal on anti seize. I asked a friend who rebuilds racecar engines professionally and got a long discussion about the relative value of different compounds, including an interesting...
www.chryslerminivan.net
The coating is a one time use as I understand it. If reusing spark plugs then anti-seize is used.
