I have two rocks and lately their butts have been so dirty. My white rock's bottom is completely brown, and the bard rock's bottom has poop hanging off of her feathers. Do I need to do something, or should I leave it?
I'd either wash them, or trim the feathers off around their back ends tp make the birds more comfortable. You might want to take a close look at them to be sure they don't have Vent Gleet or something else going on. https://www.backyardchickens.com/a/vent-gleet-aka-nasty-chicken-butt They are acting fine otherwise, are they laying?
A worm infestation can also cause their backsides to be dirty...have you recently de-wormed your flock ? If not it may be prudent to treat them for any worm problems...