Last night, I made a Peperonato pasta sauce. It was delicious, but when I added the balsamic vinegar at the end of the dish and then cooked it off (leaving a sweet reduction) it left the whole sauce a rather disgusting brown color. It was just me and my wife, so it didn't matter too much, but it wasn't appetizing to look at it, and I'd have balked at serving it to guests, even though it tastes great.
How could I have fixed this dish? More generally, how can I correct unpleasant coloring in dishes?