'Graying'?

hi everyone, can anyone please explain why my chestnut TB horse is 'graying'?  thank you 

(4 year old Chestnut (Graying) Thoroughbred Stallion)

I believe it means they carry the Gg or grey gene. I believe if the horse is born with GG, they will be born grey, but if it is Gg, it is like a slow fade. Message me for more questions, otherwise there is a helpful guide in the help center, and I believe other EVers have written more in depth explanations?

Ariel's Song is right, it means your horse is carrying the grey gene (Gg / GG). However, there is no impact on the greying process having a homozygous or heterozygous copy of the gene. 

All horses are born with their base coat, i.e chestnut, and will slowly fade to grey from 3 years old, they will be fully grey by the time they are 15. 

Thank you both so much I really appreciate it