Grey horses go through phases as they lighten up. They are born a solid color, and can either have a white mane and tail, or dark mane and tail. Usually with a dark mane and tail they take longer to lighten up, and very rarely you get a grey horse that is born light, and by the first shedding of the foal coat is completely grey. My mare will most likely always have a black mane, and black and white tail (her mom still at 12 years old has a darker mane and tail), but a lot of grey horses end up completely greying out, including the mane and tail. As smorbs said, not all horses end up getting flea bites, the horse actually has to have the gene modifier for it, and from what I've read, if the grey parent(s) isn't flea bitten, then the baby won't end up being flea bitten. Also, some horses don't ever grey out completely, and instead retain an almost varnished appearance with some parts staying darker. Grey is definitely a unique color to study.