Thankfully, cleaning a grey horse is far from difficult and will just take a bit of extra time and scrubbing.
Would you believe that the very clean horse in the photo above got that way through a shampoo bath alone? It doesn't take any special tricks!
Preferably, you will bath your grey horse the morning of the show rather than the night before. If you bath the day before, you will end up with some dirt on the coat even if you blanket most of the horse. Bathing the morning of the show ensures that your horse will be as clean as possible, unless you are unlucky enough to have your horse lie down after its bath!
I like to use a whitening shampoo on grey horses. There are lots of different kinds out there and which one you use will just depend on personal preference.
To give a good shampoo bath, first get the horse completely wet using a hose. If your horse has a yellowed tail, massage some shampoo straight from the bottle into the tail and let it sit while you bathe the rest of the horse. Pour some shampoo into a bucket and then fill the bucket with water. Use a sponge to cover the horse with soapy water, rubbing it in as you go. Once you have covered the entire horse (how much of the head you can do will depend on the horse), use the hose to rinse all of the soap off, making sure to rinse it out of the tail, too. Once you have rinsed everything off, check the horse over for any stubborn stains (the legs and belly tend to be the worst). If you find any, pour some shampoo straight from the bottle into your hands and scrub it into those areas.
If it is cool out, bring along a cooler to the wash rack and put it on your horse right after the bath to keep him warm.
Towel off the legs and try to let them dry as much as possible before putting the horse in anything more dirty than a stall full of clean shavings. Wet legs will attract dirt! If your horse likes to roll, consider tying him safely or keeping lots of hay available.
If your horse lies down after his bath and you don't have time to give him another one, you can remove the dirty spots in a few different ways. One option is to wet the area with a sponge and then scrub in a drop of shampoo before rinsing it off with the same wet sponge. Some grooms swear by diluted alcohol for removing spots, while others use a commercial spot-remover. Figure out what works best for you and stick with it!
That's all that you need to do to make your horse sparkling white. There's no secret product or method; clean hair is clean hair!