I use a product called 'Bling It On Pony' which is a white powder that you add water to and make a paste. You paint the paste onto the socks, leave it to dry and then brush off for crazy white socks. I have also found it really good for cleaning up skin issues like mud fever and cannon crud.
Wash the sock with a dandy brush and some whitening shampoo, then when dry use baby powder or maizena, sprinkle over sock and brush it even. Tip: make sure it's all dry when you put the powder on, otherwise it makes it patchy...☺️ Good luck!
Mix up a paste of talcum powder and water and paint it on the night before a show, let it dry. Use old socks with the toes cut off to cover it once dry. The next morning brush the talcum out and they look amazing!