Okay I feel a little conflicted on Snowfire. As a teen I felt like the horse should be white in the adaption and there was no reason to be have anything different than the cannon version. However looking at the movie vs the book there is something likable about Brom in his black clothing with a hint of blue riding a black horse. It visually looks better.
Compare this image from the movie with the following fan art:
However due to the name of the horse there is no easy way to simply change his color. It would need a different name, or need to be a different horse, or need its color altered by magic (which could be done for stealth reasons). Also I have no problem with Roran having a white horse.
As for the book according to the IC wiki:
“That's a beautiful animal.”— Brom
Snowfire was a magnificent white stallion, purchased by Brom in Therinsford. Although Haberth, Snowfire's owner, was hesitant to part with him, Brom promised to treat the animal "as if he had been sired by Gildintor, the greatest steed of legend". Reluctantly, Haberth agreed to sell Snowfire for two hundred crowns.
After Brom's death, Eragon took Snowfire as his own, determined to fulfill Brom's promise to Haberth. Eragon was forced to part with him in Du Weldenvarden but Snowfire was taken well care of by the Varden.
After the Battle of the Burning Plains, Snowfire still belonged to Eragon, although after he wed Roran and Katrina, he gave Snowfire to Roran, most likely because he felt he would prove more useful to Roran. This may also be because Eragon has Saphira and the speed and strength of an elf.
Snowfire has served Roran in his battles for Nasuada such as when he faced the soldiers who felt no pain: holding his own and managing to kill a few soldiers. Roran has at times, though, wished that his horse were a different color, such as a nice bay or chestnut, as Snowfire does tend to stand out in the battlefield.