I have a 3-4 inch Koran that has the same brownish appearance on the face. It brown between the eyes and the snout and I don't know what to do with it. My uncle has a Koran that is about 6 inches and his suffers from severe head and lateral line disease where the skin begins to erode away from the fish's head. You need to use vitamins like zoe or selcon and feed your koran a variety of frozen foods, flakes, spirulina, and seaweed selects. I just hope this coloring goes away on my angel by using the vitamins and other things. Good LUck