Spark Plug Color

Those are lead deposits.

One of the reasons to avoid 100LL if possible.

But nothing out of the ordinary or unusual.

No thermal paste on the plug threads?

Regards........Chook.

https://decalinchemicals.com/products/decalin-runup-fuel-additive/

Unfortunately  we can´t find Decalin in Brazil and our mogas is with 27% alcohol.

Only option AVGAS which price is 86% more than mogas. 

Thanks Arnaldo

You can order Decalin online from Aircraft Spruce.