Well I have a horse with white legs and the night before the show I will will take a bucket of water hot enough that I have to wear rubber gloves down to the barn. Then I take towels (that have been washed so that the color doesn't stain my horse) and dip them in the bucket, by that time the water has cooled off because its like 25F here. Then I scub from side to side and when the towel gets dirty I rinse if off. The key is to not get your horses legs dripping wet. You use the heat to get the dirt off/raise it to the top of the hair. Then when the legs dry I rap them and put her in her stall.
If your horse's legs are wet, let them dry then use a brush to get the mud off and then you can do this to get all mud off the skin.
It takes some time but It works wonders and it keeps her from getting sores from mud.
Hope this helps