Car is Running lean?

[MReedBMWM3]

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So I replaced my valve cover gasket over the weekend, during the job we inspected all of the spark plugs (that had been installed in Dec. or Jan) and noticed that the burn on the end was sort of a yellow, not brown. My mechanic said that's a sign of the engine running lean.

What could be causing this problem?

 

[epj3]

http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

That describes what I was thinking... all my spark plugs have been a yellowish/brownish color...which as far as I know is supposed to be normal.