i use a bog standard cheap bar of soap and scrub, stains lift easily and you get a shiney clean horse.
greys and whites are a pain in the ass, find most like to be dirty. if your lacking time then buy some fancy whitening shampoo, but a good old scrub does the trick, and if you keep ontop of the tail you shouldnt have a yellow stained one, i find in the summer once a week just the end works, and in winter twice to 3 times a week depending on where your riding (rubber schools kick up alot of dirt i find)