Changing of the colour of your cat's nose is just likely to be a change in the pigmentation as they get older. A darker pigment is actually usually healthier since it offers more protection from the sun. However as long as it doesn't get an ulcerated or raised and bleeding appearance that doesn't heal, then it is likely fine.
Other causes include possible previous trauma and healing (e.g. a cat scratch or fight wound).
In any case if you are at all concerned, you should have it checked out by your regular vet.
Martin Slome DVM
Centre Street Animal Hospital
7700 Bathurst Street Units, 40-42
Tel. (905) 771-9855