Chickens are getting dirty :(

[cerealrock78]

Two of my chickens out of four are getting dusty and gray. They're light brahmas. And thier feathers are supposed to be WHITE.
Now I'd say they're gray or brown chickens if you ask me.I'm not saying they're super dirty. Buuut I'd like their feathers to be pure white and soft, just like they used to be instead of dusty and grainy. How can i wipe the poop off thier butts, keep thier feathers white, and keep their feet clean and keep it that way and still let them free range? Please help!

 

[youngchooklover]

Don't think you can stop them getting dirty my cute little silkie who is white keeps on getting dirty.
I was used to having a clean little silkie but those days are gone.