Grey is a modifier and is known by (GG) for horses that are homozygous for their grey gene, or (Gg) for horses that are heterozygous for their grey gene.
While there is not a distinctive difference in how quickly a horse grey's out, usually at the later stages of the horse's life either choice will eventually be "white". Horses that are (GG) will always produce a foal that will also be Grey. Horses who are (Gg) have a 50% chance of passing on their Grey gene and then that foal will also be Grey.
Grey is best described as being the eraser on a pencil, it takes color away. In humans, no person is ever born with grey hair, they start out as a Base color; Black, Red, Blonde, or Brunette. As the person age's they get more grey hair until they are eventually white. The same with horses. Each horse starts out colored; Black, Chestnut, Bay, Bay Roan, Red Roan, or Blue Roan. Any of these color's in Percherons can be erased out by Grey. Also to have a Grey, one parent must be Grey. Grey cannot skip a generation. It also does not matter if your horse descends from Grey's. If they are not Grey themselves they cannot produce it unless bred to another Grey.
Grey is found in many lines of Percherons. Being the 2nd highest registered color In America, it is no wonder why. France has capitalized on heavily breeding for Grey horses for years. While it is not proven, I strongly believe that most horses located in France are homozygous Grey. Although Black is making a comeback in popularity. Some Percherons in France could easily still be Blue Roan as well. Many Grey's in America are doing exactly that, and hiding these colors underneath their Greying coat.
Interesting points about Grey's is that they tend to be more tolerable to heat due to their light coat. They also do not attract flies as badly as a dark horse would. There is also a difference between fleabitten Grey's and Dapple Grey's. It would seem that the American Percheron is losing their Dappled Grey's and more and more Fleabitten horses are being seen all the time. A shame, as Dappled is my favorite of the two choices.