I've had this little problem crop up from time to time, dirty head feathers. How do they manage to get soiled heads, anyway?
But sorry to disappoint you. I've tried washing them with a wash cloth dampened in sudsy water, and it did very little to improve the dirty heads.
I'm afraid to have to tell you that the only solution is to wait for yearly molt when the soiled feathers will be replaced. Is The Coop in the northern hemisphere? You won't have long to wait for clean, new feathers if it is.