I can't really advise what to tie to - so many things are a safety issue. I don't have a hitching post on my property (yet) when I bathe, I tie to my basketball hoop (set in concrete) on my concrete drive. The main thing is finding something safe, the horse can't pull up and drag (like a fencepost), or hurt themselves on.
As far as the bathing, on my mostly white horse I like quiksilver. It works better than anything else I've tried. I also spray all my horses down heavily with showsheen as they are drying, to repel stains and make future dirt come off easier, works a charm but beware of riding bareback too soon after that *lol*