The Grey Modifier
The grey modifier isn’t an actual colour; it is the loss of color. Grey horses begin life normally coloured and progressively lose pigment throughout their lifetimes, retaining dark skin colour. The base colour of a grey horse can be any coat colour or pattern.  In general, foals are usually born a few shades lighter than their adult colour, but grey horses are born dark. Percheron breeders often say, “a black horse is born grey, and a grey horse is born black” (Sponenberg, 2017). Because of this dark coat, it may not be possible to determine the foal’s base colour (chestnut, bay, brown or black). In addition to a darker foal coat, an indication a foal will turn grey as an adult are rings of white around the eyes often referred to as “grey goggles”.
(Click on the photo and you will see a larger gallery)

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The grey coat colour is due to the presence of a dominant gene (G) at the grey locus on the twenty-fifth chromosome. Grey is epistatic to all coat colour genes except white, which means the grey gene overrides normal coat colour. A grey horse must have at least one grey parent. In a horse of more than one colour: paint, tobiano, overo, pinto etc), the grey colouration will act upon the solid colour and leave the white areas alone. Eventually, the horse will look white all over, except when it rains or in good lighting. These “ghost paint” horses are hard to spot, but once you have seen a few, you’ll begin to recognize them.

Ghost Paints/Pintos

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As each coat sheds out, a grey horse will become lighter and lighter. The speed with which a horse attains a fully white coat is extremely variable, but it is inherited. Because greying time is hereditary, some horse breeds were selectively bred to turn white swiftly such as the Lippizaner horses and Kladruber (Danish Royal Horses), while the Percheron draft horses take much longer to whiten.  The underlying coat colour also plays a role in the length of time required to lighten simply because a lighter horse such as a palomino will appear lighter faster than a dark bay horse.

On an individual horse, the areas that lighten first are also variable but there is a general pattern that most grey horses follow. There are always exceptions to this generalization.

BEGINS:
Face (except forehead, nasal bones & lower edges of the cheek)
Tail -usually begins in the lower half of the tail
Coronet- some grey horses have a pale ring around the coronet
Throat- jugular notch (the distinct groove where the throat meets the head)
Chest
Elbows- behind the elbow
-At this point dapples will appear if a horse is prone to dappling along the shoulders, sides, and neck. The dapples spread and eventually all that remains dark are the knees, hocks and hindquarters which lighten last (Kathman, 2012).

Some horses remain partially coloured all their lives, and some never achieve white. Horses with facial markings, such as a star or blaze, will start to grey outwardly from the marking. The blaze will spread as the face lightens.

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Not all horses dapple, many simply lighten progressively and more uniformly over time. Some horses have areas of white amidst darker grey colour. A Thoroughbred named Tetrarch was known for these white spots and for reviving the dwindling number of grey Thoroughbreds (Gower, 2016).  A discussion of grey horses in racing history can be found here:

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I proudly own & ride a rescued grey Off-the-track-Thoroughbred (OTTB) named Ashe.

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“Grey horses progressively acquire white hairs throughout the coat as they age, and the greying process (rate and location) varies from horse to horse. Some horses will grey first in the mane and tail hairs while others will lose pigment last in the mane and tail (Sponenberg 2009). Additionally, some horses will retain small flecks of pigmented hairs to display a ‘flea‐bitten’ appearance , while others will retain pigmented hairs that outline areas of depigmented hairs to display a ‘dappled’ appearance. Furthermore, some grey horses will develop skin depigmentation (similar to vitiligo in humans) in addition to hair depigmentation (Sutton & Coleman 1997). It is speculated that some of the horse to horse variation in the greying process is likely due to modifier genes that have not yet been investigated. ‘Grey’ is said to be epistatic to all other coat colours and patterns, as horses that inherit the grey duplication will eventually lose pigment as they age, and any underlying coat colour or pattern will be masked by ‘grey’ ” (Thiruvenkadan, Kandasamy, & Panneerselvam, 2008).

Flea-bitten greys are horses with small flecks of a darker colour intermixed in the coats. The mechanism for this trait is unknown although most flea-bitten speckled horses are heterozygous (Gg). A flea-bitten horse bred to a non grey horse may result in offspring that are flea-bitten indicating the trait may be separate but linked to the grey color (Bailey & Brooks, 2013).

Of note, grey horses are prone to a type of melanoma which is not cancerous and does not results from exposure to sun. These tumours grow all over the body and are common in mucosa (mouth, rectum/anus, sheath), but can be found anywhere in the body. They normal pose no threat unless the tumours block major blood vessels, grow on vital organs, or interfere with quality of life for the animal.

Grey horses with black manes & tails

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Grey horses with white manes & tails

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Grey horses with black manes & white tails

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Pure grey or white

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Foals & Yearlings transitioning to grey

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Grey Pintos

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Half tails

It is not uncommon for the tails of grey horses to whiten unevenly forming a “half tail”

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Dappled Greys

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Interesting transitions

Horses such as duns, or light chestnuts often grey in an amazing variety of shades. Rose greys are chestnuts that go through a light pale red phase- hence the term rose grey

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Greys in groups

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Freeze brands

Mustangs are identified by a freeze- brand on the neck. It normall leaves hair white. They are visible in grays, just more challenging to see.

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Not Greys

Many coat colours such as roan, grullos, and cremello/perlino may be mistaken for grey. Roans and grullos retain dark heads throughout their lives. Cremello and perlinos have pink skin- especially noticeable around the muzzle.

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Flea-bitten Greys

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References
Bailey, E., & Brooks, S. (2013). Horse Genetics (2nd ed.). Boston, MA: CABI.

Gower, J. (2016). Horse Color Explained. Brattleboro, VT: Echo Point Books & Media.

Kathman, L. (2012). The Equine Tapestry (Vol. Volume 1- Draft and Coaching Breeds). Charolette, NC: Blackberry Lane Publishing.

Sponenberg, D. P. B., Rebecca. (2017). Equine Color Genetics (4th ed.). Hoboken, NJ: John Wiley & Sons Inc- Blackwell.

Thiruvenkadan, A. K., Kandasamy, N., & Panneerselvam, S. (2008). Coat colour inheritance in horses. Livestock Science, 117(2), 109-129. doi:https://doi.org/10.1016/j.livsci.2008.05.008