OK... We have a little confusion here.
We are not looking for the color of any metal part of the plug.
The color we are looking for is the color of the ceramic inside the plug....the part of the plug inside of the metal threaded portion of the plug... on a brand new, never used plug it is refrigerator white.
As it runs it should color up from pure white to some shade of tan or light brown.
No part of the plug ceramic on a decent running engine should be black.
No part of the plug ceramic should be "glowing".
And the key is not to pull it out in a big hurry before it cools off.
The key is to shut the engine off as soon as possible after the full throttle run so that idle carburetion mixture doesn't leave a color different from the wide open color.
In other words... run it hard......... get on the brakes and KILL the motor... no idling!.
You can now let it sit for a month if you want (without running) and pull the plug and see how it was running while you were flogging it.
Try it again... Unless you remember the color of the ceramic.
I also want to know how old the plug is.