1. Halter horse hooves always are black, no matter the natural color.
2. All white socks on a performance horse are natural/clear colored.
3. Any black hooves then all are painted black.
4. Stripey hooves can go either way.
5. Trend preference. If you want to be trendy and put black on white socks for a performance horse, you can, but you are at risk for hilighting the movement of the legs - for good or for bad. Putting black polish on white socks always runs the risk of drawing the judge's eye to the leg and enhances what leg movement they have.