What do you mean conceal a transplant?
Conceal the donor scar? Any procedure done by a good surgeon should result in a scar that is undetectable if the hair is grown out long enough, regardless of the color.
Conceal the fact that it's a transplant? Again, a HT should be undiscernable regardless of the color if done by a good surgeon.
That all being said, certain colors do better in adding to the illusion of density. The lighter the hair color, the weaker the contrast between scalp and hair. This will give the appearance of more fullness. So yes, salt and pepper hair should be great especially with the grey elements.