You can breed a Champagne pitbull with a white pitbull and some times get grey or a blue.
How Did They Make Gray Pit Bulls? What Color Were The Parents And Why Is Everyone Calling Them Blue?
Parents were of black, red or cream. Has to do with color genetics
A Red Nose and a Blue Nose = Silver/Grey
Greyish colored pits are blue pitbulls
Gray American Pit Bull Terriers are usually referred to as "blue". This color is caused by a genetic recessive mutation that "washes out" or "dilutes" black pigment to gray. The only way you can get blue puppies is to breed two dogs that both carry that recessive gene for dilution, OR to breed two blue dogs or a blue dog to one that carries the dilute gene. Since it's a recessive gene, a puppy must inherit a "copy" for that gene from each parent if it's going to be blue. If it gets any other gene for color other than blue, it will be that color instead.
To make a very complicated answer short, it's genetics and blue is a dilute gene and occurs most often in breeds that have a heavy concentration of black since the blue is dilute black. Both parents have to carry the blue gene no matter what color they are and whether or not any other genes are carried that are more dominant matters as well. Blue is just what we call the color. There aren't really any 'gray' dogs. It is always called something else which more accurately describes it (like silver and charcoal and steel are also terms used to specifically describe the color however each breed has their own descriptions and allowances for color, because of the health problems associated with blue it is frowned on by many breed clubs and not a desirable color in many breeds though that's not always the case and the genetics leading to the color are always a factor). Blue dogs also suffer from more skin problems usually as color can often be linked to other genes that have to do with health and many blue dogs suffer from something called blue gene allopecia (hair loss).