The patterns you are seeing are called "piston wash", a term you'd see very often on snowmachine racing forums, where wash is as important as the plug color in determining the correct jetting for the snogo for the given conditions.
From looking at the pix, you are jetted nicely. Is this the same saw you showed the coffee colored plug a few months ago? If so you are dead on.
I just saw the photos of your plug. You are dead on. The carbon building up is because the saws aren't running full throttle all the time, and as such are running cool. If you want to get a really accurate plug reading, run the saw in a cut for as long as you can at full throttle, then shut it off while still in the cut, at full throttle. Do not let it idle. You want to see what the plug and piston look like when they are hot. Prolly try to find a log you can saw in half so you can run the saw for 1-2 minutes to burn off all that carbon on the plug. Won't happen on the piston though, but you'll see what it all looks like.
Go here for some good photos of lean, rich, and correct jetting:
http://www.bikemanperformance.com/tech_detail.php?ta=4
The fellow who showed the badly scuffed piston will find his under "lean photo".