The color to indicate how rich or lean it's running. Plugs used to look light gray when gas had lead in it, you're looking for a tan to light brown most likely. You can look it up on the internet to see examples. Really dark or black means you should try leaner jets, I usually go a couple sizes at a time- for example, if you have 69s, try 67s. My Mustang and truck both have Holley 80457 carbs(600, vacuum secondary, electric choke), the truck engine is closer to stock and the jets are factory, the Mustang is a little stouter and I leaned it out 3 sizes.