While gray is visually a distinct coat color, it is genetically a pattern that is imposed over other colors. Horses are not just 'gray'; they can be bay grays, cremello grays, silver grullo roan grays, etc.
Gray is a pattern that is dominant over all other colors. For that reason, when you see a gray horse, it is impossible to tell what genes it has. A gray horse's foal color should be noted for this reason.
The gray gene (G) is dominant and thus will not skip generations. Two non-grays can never have a gray foal. Gray in horses is a bit like gray in humans-- the hair will lighten with age. In horses, the graying usually starts when the foal coat is shed, although the graying can start later when the horse is a few years old. Some horses will gray very quickly, and some will gray gradually. Eventually, the horse's coat will turn white.
While the horse is turning gray, it may develop odd patches of lighter hair that look like large spots. These are called "Watermarks". The graying process will even the horse's coat up and make these dissapear as the horse ages.
Since the graying process creates many shades as the horse gets lighter and lighter, there are many terms for the different 'shades' of gray. However, unlike shades in other colors, the shade will not remain for long and will lighten with age. "Rose Gray" is a term used to describe bays and chestnuts as they are turning gray and are a slight pink color; "Steel" or "Iron" is often used for black horses that are a deep gray color; and "dappled gray" is used on horses that have dark dapples of their base color that are visible. A "White gray" is a horse that has completely lightened
When a horse grays, the pigment is gradually removed from the skin and hair and is deposited in the gut and intestines. For this reason, grays are more likely to get melanoma (lumps of pigment granules), especially under the tail near the rectum (intestines)and on the face where pigment is deposited.
Gray horses can be born any color, as the gray pattern will show up on a horse having any base color or genes. A gray foal could be born cremello, black and white tobiano, an appaloosa leopard, etc., but the gray will eventually 'take over' and turn the horse gray. For that reason, people who breed for Appaloosa or Pinto coloration should avoid grays. Because gray is a dominant color, breeds can consist entirely of grays. An example is Lipizzan or Lipizzaner horses, a breed that is almost exclusively gray. A rare bay or black, the offspring of two heterozygous grays, is occaisionally born and is considered good luck.
Very good info in the article posted except for this statement--
"When a horse grays, the pigment is gradually removed from the skin and hair and is deposited in the gut and intestines. For this reason, grays are more likely to get melanoma (lumps of pigment granules), especially under the tail near the rectum (intestines)and on the face where pigment is deposited."
That is inaccurate. Grey horses typically do not lose pigment from their skin.
The greying action-- hairs gradually growing in white-- is because as a grey horse ages, pigment (melanin) no longer travels from the skin up the hair shaft in more and more hairs. The cells in the skin are still producing pigment-- but the genetic action of grey causes the pigment to no longer enter the hair shaft. This disruption of melanin dispersal is thought to be why grey horses are more prone to their particular type of melanoma-- the pigment collects in the skin and is basically being overproduced because it has no where to go, and can form tumors.
I have a grey arabian mare, who's mom is a flea bitten at 13 years of age, and who's dad is a bay. She at 4 years old had an almost flea bitten looking head, is definitely lighter than the body, but the legs and mane are completely black still, as is most of her tail. Her mom retained a pretty dark mane and tail herself. My horse's coat is also very very dark still, though I know that in a few years she will be lighter. It is very true that the lightening process differs in horses. I know a horse that a year ago looked very similar to my mare, but now is almost completely "white". It is a bummer in one sense that they lighten out as they age, but at the same time, it makes for a more interesting horse, to look back at pictures over the years, and see the horse when it was very dark, and watch as it has lightened up. I've read in a color genetics book that you can generally tell relatively accurately how quickly a grey is going to lighten up. The head mane, tail, and legs are generally the first to get light, and as soon as those lighten up, the body starts lightening up quicker. But we shall see with my horse, as it also said that generally by the time a grey is 3, it should have a considerably lighter head, and by the time its 4 or 5 should be getting lighter in the body, and my horse at almost 5 has a light head, but still dark mane, mostly dark tail, black legs, and a very dark body. She does have black skin though, as that had to be recorded when her papers were being drawn up, though I'm not exactly certain what difference that makes whether the horse has pink skin or black skin. If anyone knows, I'm definitely curious about it.
This is why I love greys - they change colour so much its like a new horse every year.
My mare was born dun, went dark donkey grey, then a lovely dappled grey and ended up TOTALLY white. Was awesome!
My grey pinto is doing the same thing - its neat seeing what colour he will go each season.
He may have never dappled, burt almost all grey horses are born a normal dark base color and turn white as they age. Like this mare, who resembles your Bear-- she was born dark bay/brown. Picture below is winter coat age 14.
I don't have an actual photo of her at racing age, but from her owner's descriptions she was probably at a greying stage similar to the horse pictured below when racing.