How can I fix an unpleasant color in a dish?
There are things you can do to tweak the colors, but generally it's easy to move towards brown and dark, and hard to move towards a pure shade and light.

You can use food coloring, spices with a lot of color (like turmeric). You can lighten with cornstarch, flour, and dairy. (thanks satanicpuppy)

If presentation is important to you, you can buy ingredients that make it easier to get the colors you want in the final product. For instance, next time you make the dish you could use white balsamic vinegar and a little more sugar.