I’ve noticed that any fabric that my head comes in contact with discolors after a period. Pillows turn sort of yellowish, even light-colored leather discolors. I wash my hair daily, and I don’t smoke. I notice that my wife doesn’t have this problem. Is this something odd, or is it normal?
Do you work around lots of grease (say, fast food)? Do you dye your hair?
Maybe since you have shorter hair than your wife (I assume), your scalp is more exposed to chairs and pillows. Maybe you have oilier skin than her.
That’s a lot of maybes. 
I don’t know the answer… all I have is theories.
The yellow color is the end result from facial sweat and possibly drooling while you sleep. Think “ring around the collar.”
Some of the information here may be useful.
Good information. I do eat spicy foods, onions, garlic and the like. So perhaps my body chemistry is out of whack.
Are you Brittany Spears?