As historical accuracy is so highly esteemed by the people on this forum, I thought I should share this with you: An international team of scientists has investigated the colors of horses, using samples from the dawn of horse domestication until the Middle Ages.
There are some interesting conclusions on the frequency of the different color variants. In particular, spotted variants seem to have been quite popular in Antiquity, while in later times the solid colors were preferred.
If you want to have a look - the article is free.