From About.com Chemistry: "Add the sodium hydroxide and indicator to the bubble solution. You want enough indicator so that the bubbles will be deeply colored. For each liter of bubble solution (4 cups) this is about 1-1/2 to 2 teaspoons of phenolphthalein (red) or thymolphthalein (blue). Add sodium hydroxide until you get the indicator to change from colorless to colored (about half a teaspoon should do the trick). A little more sodium hydroxide will result in a bubble that stays colored longer."
What a fun Memorial Day weekend project.
[The About.com link is from this debacle. It's still a cool project, even if the Instructable-r stole it.]