We used bougainvillea for this. But you'll need lots of them, a branch wasn't enough for us. And use the dark-colored ones. I hope you have some in your neighborhood. It's really easy to find.
Red cabbage would be the best for this, you can buy it at the supermarket, but it would be difficult to find one open at this hour. During our experiment using the red cabbage, we were able to make all the rainbow colors with varying pH values. :)
See attached for a shot of bougainvillea :)