Gray - sekelsta/horse-colors GitHub Wiki
Gray horses gradually change color as they age. When a foal, they may appear to be bay, black, chestnut, or another color. As they grow up, they turn gray and then white. Different horses may gray at different rates, so one might be fully white at three years old while another still has black hairs at 18.
To tell an aged gray horse apart from a white horse, look at the skin color on the nose. Gray does not affect the skin color at all, so gray horses will usually have a black nose. On a white horse, the skin is pink.
There are two alleles for the gray gene:
G: Gray, dominant
g: Not gray, recessive
Homozygous gray horses (those with G/G) turn white faster and are more likely to develop melanomas, as described here.
Equine melanoma
The gene for the gray coat color also causes equine melanoma. Equine melanoma associated with the gray gene is typically more benign than in humans, and tends to make annoying lumps. In game, this just decreases max health by a small amount. As the horse gets older there is a chance the melanoma could turn malignant, but Minecraft does not have old age so this will not happen in game. Some gray breeds are more likely to be affected than others, with breeds that gray out faster and more completely having a higher rate of melanomas, so there's thought to be some sort of genetic component other than just the gray gene. So far it seems that having two copies of gray is worse than just one. Other, hidden, risk factors are also heritable in game.
The gray color actually happens because the melanocytes reproduce more often than usual. In the hair follicles, after the melanocytes split too many times, they can't do it anymore and they die. After that the hair grows in white. But in the skin there are stem cells, so the melanocytes will always be able to split more. This is where the link to cancer comes from - the first step to cancer is cells that reproduce more often, and gray is the cells reproducing too often.
Research has not yet confidently identified genes which are risk factors for equine melanoma in real horses, but a study in Lipizzans has pointed out a few that may be involved.