The dark color could be because of the clear water or shallow depth. Fish tend to lose color in dark or stained water. It’s most noticeable with crappies in spring when they move up to spawn. LM bass also noticeably change color in only a few days.
Yeah, I’ve never seen one this dark in color, the few I’ve caught, (few because I didn’t really start targeting walleye til later last year) have been much, much lighter in color, grey. Not to mention this one is probably twice the size of my biggest. Interesting though, never seeen the brownish color til now, now I know.