Its no different to leaded fuels. What leaves the colour on the plug is the carbon left over from the burn if the mixture is richer than stoich i.e richer than 14.7. The reason plugs remain white on the porcelain is the mixture there is very lean at time of ignition. When you improve the mixture "Quality" you can get the porcelain to colour as well. If the mixture isn't sufficiently compressed then the heat from ignition has to get it to a stage where it will burn and that takes a lot of the energy and the rest of the burn proceeds slower than it should and require more timing to get the pressure peak at the right place and time........
