Spark plugs diagnoses

[MX3ak]

These plugs have 500 Km/miles more or less on them. Some have inner ceramic brillant white while some others are yellow almost brow.
I've googled for some help on reading these facts about plugs but all I came across was extreme conditions.
Like this link http://www.beru.com/english/produkte/zu ... ehilfe.php

I saw that yellow/brow is good on aged plugs, but since they are new, why are they different that the others that are as white as new ?

I'de like to see your personal thoughts

here's the pic

[mitmaks]

http://www.aa1car.com/library/reading_spark_plugs.jpg

they dont look bad except 5th and 6th (especially 6th) almost like its misfiring

[coltfoet5]

Do you have oil leaking into the 6th from valve cover?