Hey all! Ran into this bit of info.... Hope it helps someone! "Red cabbage contains a pigment molecule called flavin (an anthocyanin). This water-soluble pigment is also found in apple skin, plums, poppies, cornflowers, and grapes. Very acidic solutions will turn anthocyanin a red color. Neutral solutions result in a purplish color. Basic solutions appear in greenish-yellow. Therefore, it is possible to determine the pH of a solution based on the color it turns the anthocyanin pigments in red cabbage juice." http://chemistry.about.com/library/weekly/aa012803a.htm
Also, Home and Garden pH Indicators That's a list of common crops that'll show pH. Oh, and I suggest morning glories, as the pH levels it has listed are right in between those that which are wanted. I believe, that the leaves from the plant will change color based on the solution (for hydroponics, just floating a few leaves in the water) but I would only suggest them for outdoor grows of course.