i didnt think they would ever change back once they have gone white - depending on colour of horse you could temporarily die it if it is for a show class or similar short period.
Going to go against the grain here but they can return to their normal colour.
My friends horse has white hair appear under the saddle area, as soon as the saddle was sorted no more white hairs came, however she had to wait until spring for the new hair to come though and the white hair then fell out and it was fine - back to normal colour.
Used to be called Grecian2000.If you use this tho it works over a period of time so don't expect to use one day before a show.Will say it worked on my welsh cob.The white hair will grow back in tho.