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Once you have run a car for a while you get to know its parts. The condition of the spark plugs is a direct readout of how well the engine is working. If the insulator or the electrodes look like any of the following, here is what it means.
Case 1: a light brown or grey insulator, with perhaps a little light carbon, means the heat range is right and the engine is working normally. Ignition timing, air-fuel ratio, compression and the ignition system are all fine.
Case 2: a white insulator tip, sometimes with blisters or electrode erosion, points to an abnormal air-fuel ratio, ignition timing or coolant temperature, or to the wrong plug grade. Fit a colder plug.
Case 3: a yellow or dark red insulator suggests the plug ran very hot under heavy load or high speed, so combustion by-products could not escape and instead fused onto the insulator, forming a conductive layer. Replace with a colder plug to keep the engine running properly.
Case 4: a sooty black plug indicates the wrong heat range, an over-rich mixture, or oil getting past the rings.
Case 5: a wet plug can mean an abnormal air-fuel ratio or compression (worn pistons, incorrect valve clearance, leaking valves), poor plug performance, an ignition fault, or gas leaking between cylinders and poor sealing.
Case 6: white-brown deposits on the side or centre electrode mean the additives in the oil or fuel are poor quality or badly proportioned. Change the oil and fuel, clean the plug, then refit.
Case 7: spots and deposits of varying degree on the insulator indicate further fouling that should be investigated.