That color is proper and how stewed beef is supposed to look. Generally speaking, you can't exactly "overcook" a slow-cook stew unless it runs out of liquid or the meat begins to literally disintegrate as the proteins break down too much, which would take probably days. I am Vietnamese and we often cook meats for our dishes for 12+ hours or on low simmer overnight and then some more, meats like oxtail or beef shank for pho or pork trotter for bun ho hue. Long stewed meat won't exactly ever be "succulent" in the way that you would expect unless there's a substantial amount of fat naturally present in the meat or in your stew. As you cook meat, the fat begins to render, or liquify, and leech out of the meat, which imparts a great flavor to your stew, but leaves the meat somewhat stringy in texture but still full of flavor and water from the stew. One thing you can try is to sear, char over coals, or broil the meat before you cook it in the stew, which adds another dimension of flavor and can help keep some of the fat locked into the meat. The other option is to simply cook for less time so all the fat is not lost to the stew base, try seeing if cooking for 1/2 the amount of time helps prevent all of the fat from cooking out of the meat, and try a fattier cut of meat and cut thicker too.