I changed the plugs on my son’s 2009 Sonata 4-cyl today. One of the coils (second from passenger side) was discolored; it’s the right-hand one in the photo.
The plug from that cylinder had some brown discoloration on the insulator and on the metal body below the hex; it’s the RIGHT hand one in the photo.
(The left-hand plug in the photo is the second-worst discoloration of the four. The best one was almost completely clean.)
What caused that discoloration, and is it anything to worry about?
I doubt that’s anything to be concerned about. The spark plug positions are not identical one to the other. It’s not configured as an infinitely long line of spark plugs in other words, some are closer to the center of the engine, some are closer to the ends. So some plug positions will tend to have more heat discoloration and get more gunked up than the others. This is true of the part of the plug inside the cylinder too, as the air and fuel intake path into the cylinder is not identical, plug position to position. That said, if you think there’s a significant difference it makes sense to look at each plug closely, use a magnifying glass if necessary, to make sure there’s no cracks in the porcelain or carbon tracks indicating the spark is jumping around. And the tip, make sure that isn’t looking different plug to plug or the gap is different. The gap size is closely related to the power in the spark, and the amount of electrical heat produced. Bigger gaps command more electrical power delivered to the plug. That can stress the wire, connector, and the coil.
It looks like the heat is higher on the middle two cylinders and there’s nothing you can do about that.
What is gap on old plugs?
Thanks for the replies. I will stop being concerned.
I should have mentioned that the Sonata has 91.3K miles on the odo. We got it used about five years ago, and I presume this is first change of plugs (scheduled maintenance per the Manual, just a little early for convenience).
I did not measure gap on the old plugs before I put them in the trash. The electrodes looked pretty good (see photo) and the gap was not eyeball different from identical new plugs (which I did measure).
I had a lot of fun (and saved my son a bunch of money; sorry, you professional mechanics). In addition to changing the plugs I replaced the window switch module on the driver’s door and I replaced the center brake light (hard!!). A few hours’ free time in retirement and an unseasonably warm day in December – can’t be beat. Google and YouTube are, indeed, our friends.
Ain’t retirement wonderful? 
The discoloration is nothing to be concerned about. It’s a bit of oil vapor being baked onto the plugs and coil boots. It is possible that you have some seepage past the spark plug tube seals, but it doesn’t appear from the photos to be serious.
Congratulations on your success with the new hobby.
Oh, I’ve enjoyed retirement, for sure. :>)
But this is not a new hobby – just the first chance in a while to do something in my skill set – i.e., I don’t know enough computer science to fix the things that go wrong with modern cars, and my body doesn’t want to work under the car any more.
My first amazing car repair experience was “rebuilding” the carb on my brother’s '67 Mustang around 1972. As I recall, the kit had a new needle valve, and maybe a float. I had no idea what I was doing.
I’ve done lots of points and condensers, spark plugs, water pumps, brake shoes and pads, a couple of radiators, … Most amazing repair was replacing the A-arm on a '74 Volvo 245 after a curb impact. I had to hacksaw through the big pivot bolt because it was to bent to pull straight out.
This forum has been a help to me many times over many years. Thank you, all.
LOL, I’m not a computer guy either. I try to just remember that modern internal combustion engines make power exactly the same way that carbureted engines did. They just use sensors and injectors to meter fuel instead of fuel bowls and Venturi’s principles. The biggest difference I find is in EVAP systems. Those things are the bane of mankind! 
When I started driving, oi; changes were at 1200 miles, spark plugs got pulled and cleaned at 5000 and replaced at 10 and when I got my first job at a gas station I will always remember the “37” Pontiac because it had 37 grease fittings. Also if you got 15000 miles on a set of tires you were doing good. Also, you added a can of top oil every second gas tank fill.
