I put 4 new plugs BOSCH in my mondeo, ok to start with but less than 100miles later TWO of them were arcing like f**K, leaking up the side of the plug, I had 10mm Kerr Nelson leads on as well, swapped to Motorcraft, been perfect for three years now.
You look for greyish streaks up the side of the porcelain on the plug, a "stain" around the base is normal, and light brown in colour don't worry.
tabetha