I know a lot has been written on this subject, but I'm struggling to read the plugs on my AU2 XR6 VCT. I just replaced the plugs - after way too long - and I am seeing inconsistent colour on the side electrodes. The Autolite 985s that came out show a range of colour change location: from right at the tip to right at the base (see pic attached). This is supposed to mean that the plugs range from too cold to too hot. But they are identical spark plugs. I checked the ignition leads and coil pack, and they are excellent: all three circuits (1-6, 2-5, 3-4) are showing 12kohms resistance, so that is not the cause. And the colour of the insulators was normal - white one side, brown/grey the other. The car runs fine, but I am interested in finding out if I have a problem looming. Is it maybe indicating lean in one cylinder and rich in another? Thanks for the help.

Your centre electrodes look fine, this is what you should be focusing on. I wouldnt worry about the grounds in this case they don't appear excessively burnt. Perhaps the orientation of the plugs in the combustion chamber are having an effect? IE the ground electrodes are facing a slightly different way?

That would make sense. Thanks very much.