Greying factors are genes which result in the mixing in of white hairs with the original colour. This may be progressive (the grey factor G), or non-progressive and restricted to certain body parts (the roan factor R).

Greying factors (G)

The grey factor is a dominant gene, meaning that any horse with at least one grey gene will always eventually turn grey. The grey gene will "override" any other colour genes, meaning that regardless of basic colour or any other modifications such as dilutions, the hair will eventually all turn white.

The effect of the grey gene is akin to premature aging of the hair – the hair loses its original pigmentation and becomes colourless – it appears white. The colour of the underlying skin will not change. The process of greying is progressive, but the rate of progression is highly individual.

Foals are born in their basic colour, complete with any visible modifications provided by genetic factors such as white patches caused by patching genes; the first sign of greying is usually visible as a few white hairs around the eyes shortly after birth. Some foals may start to show a considerable amount of grey hair when they lose their foal coat; others may only start losing their colour at three or four.

How long the process takes to complete is also highly individual, with some appearing completely white at 7-8 years of age and others taking up to 20 years to lose all trace of their original colour. The mane and tail may progress at a different rate from the rest of the coat – in some they turn all white faster, in some horses slower. Progression of greying will sooner or later make it impossible to discern a horse’s original colour.

Phases of greying

Depending on original colour, speed of greying and other factors, a grey horse may pass through several stages of greying during its lifetime. There are terms applied to some of the most common phases:

• Steel grey: a black or dark brown horse that has some white hair mixed in throughout the coat, but the coat is still dominated by the basic colour, the resulting appearance is dark grey.
• Rose grey: a chestnut or bay horse that is in the process of greying, but still with a considerable amount of original colour giving the coat a pinkish sheen.

Roan factor (Rn)

• Dapple grey: a phase in which the coat is dominated by circular patches of almost all white hair surrounded by margins of darker hair (containing more original colour). The spotted appearance is known as dappling.  
• Flea-bitten grey: this appears after the coat has (almost) turned pure white and consists of many small darker spots emerging on the white hair, the spots are usually brown in colour and may accumulate until the horse appears almost rose grey in colour. Certain breeds (e.g. Arabians) appear more likely to become flea-bitten as they age. 

A notable side effect of greying is the greatly increased likelihood of a horse developing a melanoma.

Roan factor (Rn)

The roan gene also leads to a greying of the coat; however, in contrast to the grey gene, the greying is not progressive (i.e. the proportion of white hair mixed in with a basic colour stays more or less the same) and is confined to certain parts of the body (the neck and body); the basic colour remains visible on the head and limbs; mane and tail also keep their original pigmentation. Some horses show a pronounced seasonal colour difference; the warmer but finer undercoat is generally white in colour, making roans appear lighter in winter than in summer. The roan gene is a dominant trait, i.e. to be a roan, a horse must have at least one roan parent.

• A bay horse with a roan gene will have a bay head, black main and tail and black limbs. It is known as a red roan. 
• A black horse with a roan gene will have a black head, mane, tail and limbs. It is known as a blue roan.
• A chestnut horse with a roan gene has a chestnut head, mane, tail and limbs. It is known as a strawberry roan due to the pink colour of the body where the chestnut hair mixes with the grey.