GB,
Here are some color charts to illustrate the concept:
https://www.google.com/search?q=spark+plug+diagnosis+chart&sa=X&biw=1440&bih=794&tbm=isch&source=iu&ictx=1&fir=iYoXqSLc98lKNM%253A%252CMycfoz0dCKMYOM%252C_&usg=__SIdHvbKy1tqUC2YRNsa3XPh_rjM%3D&ved=2ahUKEwjt5MaY_tDaAhXrz4MKHdHcDecQ9QEwAXoECAAQOw#imgrc=iYoXqSLc98lKNM:
The terms "hot" and "cold" with respect to spark plugs describe how fast the plug dissipates heat to the cylinder head it is screwed into. A "cold" plug dissipates heat faster and vice versa. In a property jetted engine that is not drawing oil into the combustion chamber the color of the porcelain is an indicator of the plug's ability to "burn off" combustion deposits. In other words, it is a thermometer telling you things about your engine.
Changing the jetting will change the spark plug color. However it is critically important to understand that jetting has to be correct for the engine, not for the spark plug. For instance, if excessively "cold" plugs are in an engine and the plugs indicate a "rich" condition, leaning the jetting to improve the plug color may make the mixture excessively lean, with the potential to burn valves and/or melt holes in pistons.
Plug "reading" to jet an engine is possible and is done successfully all the time but tech savvy people (I'm not one of them) do the job much more accurately with an O2 sensor in the exhaust pipe and an air-fuel ratio gauge.
Have fun!
