Why this collection is called Shades of Gray, is a mystery. Although, it probably is an Easter Egg, celebrating the novel Fifty Shades Of Grey by E. L. James.
The Shades of Gray Collection doesn't require items, but require certain colors on a horse. This means, that to complete the collection, you need to own three horses, one of each color, at the same time. You can own other horses too, that doesn't matter. The fact that they seem to be a specific gender, doesn't matter at all. This is all just to make the name more interesting rather than calling it "Blue Horse, Green Horse, Purple Horse". All horses in Derby Days are "genderless", meaning that they both can deliver foals, and breed other horses. When you succesfully breed/buy a horse of the wanted color, you will get a message about it, same as when you are trying to complete any other collection. This way, you can always be sure if your horse is the right color. If you have a horse in the right color, and you sell it, the horse will dissapear from the collection again. You need all three horses at the same time to complete it.
Items Required[]
Blue Horse
Green Horse
Purple Horse
Reward[]
Bronze Statue