I've started taking collagen powder and noticed a retardation of growth for my grey hair. Is it medically possible? Can collagen really be replaced?

Not exactly . . . Yes, while one is taking collagen powder, he may notice that his grey hair is not increasing in number or length like before, but the two events don't have anything to do with one another. Collagen that one eats is digested to make whatever proteins the body is currently making. Collagen can be partly replaced or repaired, but as we can all see with aging, the results are never as good as before.