Purple shampoo is great but it takes a while so if you do it the night before a show keeping them clean is tricky. I would invest in a Rambo DustBuster sheet. They are tight blankets that keep them clean and almost buff their coat and leave them shinier the next day. They are a little expensive but really worth it and they hold up really well
Lucky Braids shampoo and enzyme whitener spray. A bit more $ than the other suggestions but they work really well. Developed by a long time groom. Great customer service. Check out their website for more info
Use a purple or whitening shampoo, and bathe as late as possible the night before the show to prevent your horse from getting too dusty and dirty. You can also put a sheet on your horse and make sure the stall is clean before putting the horse in it. Hope this helps!
I had a dapple grey and we used to wash him in normal soap and then purple soap, the purple one works miricles! Use a brush to get the soap all the way down and also with the purple one make sure you rinse it our fully and don't leave it on too long (it will stain their coat purple so maybe first time you use it just try the tail