Nissan Repair: My nissan serena spark plug, air screws, air screw

Question
The colour for my serena plugs have two diffrent colour. The electrode wch is L shaped and the centre part is coffee tan whilst the plug base ring is black/dark grey sooted. Is this a sign of rich mixture? Whats the correct plug colour ? Its fuel consumption is 6.5km/litre city drive whilst 9.2km/litr highway. It is a 2.0 engine. Is this normal consumption? How many times should the air screw be turned out. I turned it 1 and a quarter but it seems the car runs richer. Now l have turned it 4 times out m stil to discover the effect. Also if turned out 1,25 out the oil check light flickers wen idling at operatin temperature goes off upon reving, but at 4 turns out it doesnt flicker? Pliz help the car seems thirsty. All sensors checked and correctly functioning.

Answer
First make sure you are using the correct heat range spark plugs. The dark coating on the rim of the plug indicates rich mixture at start up when the motor is cool. Secondly, you should never try to adjust any "air screws" and in fact, all U.S. vehicles have any adjusting screws sealed off to prevent drivers from messing with them, as such adjustments without the proper tools will have a negative effect on the emissions and the engine's performance. A tailpipe  exhaust analyzer needs to be used now to determine if the engine runs rich or lean and a scan tool to pull the trouble codes if any. Please take the car to a technician :)