First of all, red cabbage juice is the way to go if you are on a shoestring budget. All you need to make it is a head of red cabbage, a pot to boil it in, and some water. Slice the cabbage, boil it for about 20 minutes in the water, and you can drain the red cabbage juice from it. To answer your question, the first limitation is the color changes it produces are approximate, they give ranges rather than exact numbers of pH, so accuracy is limited. The second limitation is, if you are using it for a titration, the color change occurs after you pass the point of neutralization, so accuracy falls into play again. And finally, the third limitation would be the production of a clear solution for very alkaline substances, as the anthocyanin in the red cabbage juice has been completely reduced. Again, hard to get accurate reads on extremes with the pH scale. If pH ranges are accepatable, substance that are acidic, lower than 7 on the pH scale, turn pink. Substances that are neutral, at or around 7, turn purple, and substances that are alkaline turn a yellowish-green shade.