My current stock is just beige, because it is essentially (large) turkey necks. I took a friend's advice and slow-baked the necks, very early in the morning yesterday - the odour was a pleasant awakening for human and feline alike. Then more slow stewing in water with citrus juice (lemon and lime) added, along with onions and garlic. I stripped most of the meat off the necks (a lot more work than that short phrase conveys, and one wee cut) and returned the neck bones and flesh clinging to them. Left the soup in the pot and am giving it another bit of cooking; then will strain out bones and remains of the onion etc and reduce the stock a bit.
So I have a good glass container of shreds of neck meat - tasty but the larger pieces are a bit too hard and chewy. I have a good glass of red wine left over, and more lime juice, so they'll go in with the meat to stew it a bit. Dark beer would also be a good option. I do have a mild (bland) red pepper which I can chop up to add some colour - it is not a very pretty dish.
There are various cheats to darken the color without changing the flavor ~ a dash of worchestershire or soy sauce or maggi (teeeeeny dashes!), a small splash of coffee, or you could whip up a darkish roux and thin it with the stock & stir that back into the soup.
But since you're eating it at home & you know all the beautiful stuff that went into it, does it matter whether or not it's pretty?
I have found that adding soy sauce to all sorts of soupy things or sauces generally makes them look disgusting unless they are already supposed to be dark brown. They taste fine, but I have cancelled a number of photo shoots due to the visual result.