It's really up to you. The only time (that I know of) that there are rules for it is in appaloosa showing, where the polish has to be clear to show off the hoof striping as it's a breed trait. Many paint (APHA) show with black hoof polish on horses with completely white legs. It really is personal preference, and what you want to highlight though.
For instance, if your horse has really nice straight legs and you want the judge notice, put on black. If not or you want it more subtle, put on clear. I personally don't like black polish on chestnut or other lightish colours (ones without a black base/black legs) even if they have dark/grey hooves, as it looks too fake to me. However I think a loud black and white pinto looks sharp with black hooves. Same with a bay with strong dark points and even socks. It makes to notice the legs.
If I had a horse that had (a) sock(s) with ermine spots on it, I'd be partial to clear polish as I like the striped look the spots give.
Just whatever you do, if your horse has socks on some feet but not others, do NOT put black on the dark hooves and clear on the light. It just makes things look uneven and strange.