When I bathed my white silkies, I used Midnight White dog shampoo. It is made to take the stains out of white dogs. It is also tearless. Whatever brand of dog shampoo for white dogs (it will be blue) you get, make sure it is tearless. I thin mine is a smaller bottle. About 1 oz of shampoo to 4 oz of water in a squirt bottle.
I have a roo whose hackles were very yellow. With 1 wash it took most of it out. Do the spots with the stains first.The day I washed them, I did 29 chickens in a couple hours, I also did a permethan dip.
I started in the late morning after it warmed up. I had 4 - 5 gallon buckets. A lawn chair and an empty plastic tote for a table. Wear your swim suit.
1. Water and a little shampoo to wet them down.
2 &3. Water for rinse.
4. Dip. Permethian will take care of lice, mites and scaly leg.
I would move bucket 3 to bucket 2 when it got dirty, then refill bucket 3. My hose laying in the sun warmed the water enough to refill it.
With my left hand, I grabbed the chickens by their legs. Held their wings with my right hand and plunged them into the water feet first. Used my right hand to work through their feathers to get them wet to the skin. And splash water on top of their head.
Take them out and set them on the table, add shampoo. Do the stains first. Work through. The muddy poof may take some working through, just keep rubbing between your fingers and adding water as needed.
Rinse in #2 bucket the same way you wetted them in the #1 bucket. Repeat in #3 bucket.
I have 9 white silkies. While I was trying to catch the last 2, I had trouble figuring out who I needed, because most of them were dry already. I ended up smelling them!
Good luck!