I've had Neige (white DeT male) for about a month now, and I recently noticed a little hint of black on his head, lining his gills, and a small faint patch of the same in one of his fins.
Not sure if there's much of a difference to tell in the shots (little bugger doesn't want to stay still, and I don't want the flash to drive him crazy). He's still actively zipping about his tank and going up for air normally, eating just fine, and generally normal (including flaring at me when I sat down to look at him). The water temp is fine as well. No filter, but he gets constant partial water changes (which I'm about to go do as soon as I post this, just in case!). But I was wondering if this was any kind of sickness or maybe he's marbling like my first DeT, Tristan, did.