If you spawn in a trap horse with any sort of armor, one of the skeleton horses it spawns will be pure white. Not like a white horse, it seems to just be a blank texture.
You can use this command to replicate, but it happens with all three types of horse armor.
/summon EntityHorse ~ ~1 ~ {Type:0,SkeletonTrap:1b,ArmorItem:{id:diamond_horse_armor}}

If you try to ride the horse you see it wearing horse armour of quantity 0 which disappears if you click on it, but the horse stays white.