Here is some info I found a while back. Read through it. Your plugs look pretty good to me overall>
When observing deposits on the sides of the center electrode.
The deposits here should only be light after a lot of miles on the engine. If the deposits
are building up with a few runs the plug is too cold. Deposits of any type here can cause
electrical conduction and wastage of some or all of the spark energy.
2- The strap or ground electrode
The projection of this into the cylinder is the most important factor to the combustion.
When the arc is too close to the head surface the arc occurs in the boundary layer gas.
When the arc is in the boundary layer it is not in fresh air. The boundary layer is not
effectively scavenged. Different combustion chamber design and valve seat angles
and different porting alters the scavenging of the boundary layer. The boundary layer
is possible to be.040" thick but that is unlikely in that part of the engine. But I have
gained 800rpm's of torque converter stall in some tune situations by projecting the
plug further into the chamber. The optimum arc location depends on the amount of
vaporization that occurs around the arc location. If the mixture distribution is
improved then the arc can be further out in the cylinder. The further out it can be used
the more power the engine makes because the flame kernel is able to grow in a more
spherical fashion and more molecules are engaged in early combustion. That greatly
increases the energy within the cylinder. When tuning to test projected arc locations it
is important to reduce the ignition timing first. And one must proceed carefully.
3- The outer surface of the porcelain near the roll at the top of the porcelain
and the flat section going across to the center electrode.
This area shows the coloration of the very first part of the flame kernel. It is a zone
that gives clues to the changing burn of the flame kernel during the time of electricity
duration.
4- The sides of the porcelain down to about 3/16th of an inch above the base of
the porcelain.
This is read in conjunction with the reading of #3. Changes in color between #3 and
#4 can be used as a gradient indicator of the burn style.
5- The base of the porcelain where it meets the metal area of the body including
about 3/16th" above that.
This never gets hot enough to alter or remove deposits. Dark grey is rich side of
stoichiometric and is the common colour aimed for with Holley style carburetion.
However tan is possible and usually occurs on high power per cubic inch cylinders.
Deposits formed here are due mainly to the lower heat input to the mixture, which is
because the base of the plug is cooled by the water jacket. The gas level present here
is greatly affected by the scavenging process. If the scavenge is inadequate to clear
gases from the base of the plug a few possible conditions can arise. The gas
composition present here after scavenge is important too. If the gas is predominantly
CO2 the resident gases could add heat to the incoming charge and assist vaporization
without adding a fuel gas to the mixture. If the gas is CO the mixture will be heated
less but a fuel gas will be present. CO has a very low heat output and it tends to create
a grey deposit. If the zone is effectively scavenged the heat must be sufficient to
vaporize the fuel to a sufficient gas state, if that’s not achieved yellows or greens
appear near the base. If sufficient vaporization is achieved then light deposits of tan
etc will appear.
6- The inner surface of the body of the plug that is exposed to the combustion
gases.
This indicates the effective cooling of the body by the water. The type of carbon
deposited here can vary a lot because of what style of burn is happening. Here you
can see the effect of water temperature on the vaporization, It is through experience at
looking at lots of engines and understanding their intricacies in various weather
conditions that you learn how water temperature affects combinations.
7- The threaded area.
The threads should never be exposed in the chamber. It is necessary to check the
thread length of the plug and to determine if any thread is exposed inside the
chamber. This must be done with the cylinder head removed from the engine. If any
thread is exposed to the chamber the temperature of the strap electrode will be raised
significantly. The strap electrode looses heat to the cylinder head via the transmission
path of the threads.
The threads that are engaged within the head can still get dark deposits on them. What
they indicate is the cylinder pressure. If the threads don’t show any signs at all of dark
creeping up the threads then the engine is a slug. A good engine will blow deposit
colouration up to 2 or even 3 threads.