Jan, it really depends on how the fish heals ( what type of skin is created in the scrape) and how much of the dermis/epidermis is involved. IF the koi has a deep distribution of chromatophores then you should see some color return. But it is often a different shade as you are looking at a different depth and distribution. This would be like looking at the beni on the head of the koi and the body. Due to the skin thickness and scale effects, those beni shades always look slightly different. A scrape that healed would likely produce the same effect.
If you look at a scrap on a koi's head ( especially tancho) you can get a real lesson in how beni is packed in a minute region of skin. On the body it is a little more resilient as the skin wraps the scale and is on top and bottom. So you can expect some return of color- albeit often thin and like a mared area- always a little thinner.
One of the reasons big perfectly even beni fish are so valuable is because they still are perfect after many years of life ( no dings, dents or bumps). This is special because as we all know- shite happens!
JR