Well you need to look at more then just the color to get an idea of what's going on.
Usually the darker the richer the engine is or you might be doing a lot of idleing.
You want to see how much of the puffy black soot is on the outer ring of the plug and down the isolator.
There are a lot of sites that show dark or rust colored plugs as normal, it can also depend on fuel additives. Bone white plugs are when the engine is running very lean.
There should be color to them but they should be uniform, you should be able to see a darker ring right around the electrode that helps figure out if your idle mixture is good and if your timing is good.
There should also be a discoloration of the plug Side electrode you want the discoloration right in the middle of the elbow, to have correct timing for the motor.
25 to the right of 0 would be 25 degrees of timing. If your getting this at idle it would mean your vacuum advance is kicking in, to find out your initial timing you would plug off the vacuum advance then take a reading with your timing light.
You could have any amount of initial timing that your motor and starter can take.
Too much timing can cause very hard starts or dieseling when shutting a motor down.
Every motor is different and reacts different to timing. Ford recommended initial at 6-8 degrees, some people like 10 degrees as a starting point. The more initial timing the motor has the more power you make at the cost of drivability.
When playing with timing and spark plugs you first have to get the heat range right, motors can tolerate a huge range, but to start getting a sweet spot for a tune you have to get the plug that makes the engine run at the right temperature. Once you find a plug that shows discoloration on the threaded base of 2 threads then, you put 50 miles on it and pull the
Plugs for a look, you see the location of discoloration on the Side electrode and from that you can tell if you have too much or too little timing.
Then you drive some more and pull them and look at how carbon fouled they are.
It's easier said then down, but it helps get you in the ball park of what your engine wants. What works for one guy will not work for you.
I've been playing with it for a few years and I'm still learning.
My last plug change I went too cold on the plugs so I have some issues when the engine is cold.