Darkening in the area of neutering operation?

The area for any surgery will have the fur shaved off and it's very common for the fur to grow back a different colour, usually darker. It is purely cosmetic and causes the cat no problems although when a female pedigree cat is booked in to be spayed most Vets will offer a midline spay rather than the flank so any change in the colour of the fur isn't really noticeable, important if you want to show your cat.