Congrats on the new horse!! :) I have a faded gray paint (basically ALL white), be ready to wash your horse a LOT.
To keep my horse white in the winter- he has a winter blanket, I normally keep his tail wrapped up with vet-wrap, I brush him on regular occasions and that's it. I don't obsess with him being perfectly white, and most days he isn't.
In the summer (my show season) it is a lot easier to bath, but more work. After every ride I hose him off (no shampoo, and he's usually sweaty anyways so it's a win-win). Day before the show I do most of the prep, I wash him with a whitening shampoo (Quick Silver works wonders) really concentrating on those stained spots, but making sure to shampoo all over. I leave the shampoo in for about 10 minutes on his mane, tail and any spots.
After he is all rinsed off, I start on the tail and spray with a cowboy magic and a leave in whitener. After it's brushed and silky, I braid it and put it into a tail bag. Then I clean his face, he HATES water around his face, so I get a soaking wet rag and use that first on his face to get major dirt. I use a rag full of leave in whitener after to give it that 'glow'.
After I'm done with all of that he's usually dry enough to do stuff like clipping, braiding/banding mane. Then I put on a sleezy (it will be your best friend!), a lightweight blanket and make sure his stall is 110% clean with LOTS of fresh bedding! My horse has a run attached to his stall, but I always keep him in just the stall the day before just so he doesn't rip his sleezy or have a horse rip his blanket. I also add extra hay to hopefully keep him busy enough he won't want to roll.
In the morning the next day I get up early to do touch ups, and always plan on washing his legs again so they get that special glow. Make sure you leave enough time for the legs to dry though!
Have fun with your new horse, be prepared for lots of baths!
What kind of Leave-In whitener do you use OuttatheBlue?