High quality food does more than anything (except good water quality, of course!). The first ingredients should be some sort of shrimp or fish, not wheat or soy. I also add a supplement called "Vita-Chem" that probably helps with color. However, keep in mind that some fish just won't change. I have one boy that is only just now changed after almost 2 years! I have another boy that has changed so much in 3 months he doesn't even look like the same fish. My new girl doesn't look like she will change much, but who knows?
Also, pictures really help let you know if they have changed. Since you live with them you might not notice the changes until you look back at an old picture.