Discoloration inside head cover

[utmba95]

I'm doing a timing belt and head cover gasket, and when I got to the driver's side, the coloration on the inside of the head cover is much darker (almost black) near the back two pistons. The cam shaft and plug tube are also darker in that spot. Any idea what's causing this? The front and back spark plugs look pretty identical to me.

 

[utmba95]

Pics

 

[gaber6]

Plugs look decent to me. Maybe the airflow within the cover caused it. Maybe failing valve stem seals allowed a little more exhaust gas out there. If it ru s good I'd not be concerned.