Tonic water is slightly acidic, so pH paper will turn a slightly orange or yellow color.

Here's why:

* Tonic water contains quinine, which is a naturally occurring alkaloid. Quinine is responsible for the bitter taste of tonic water.

* Quinine is weakly basic, but the acidity of the citric acid added to tonic water overcomes the basicity of quinine.

* The overall pH of tonic water is slightly acidic, typically around 3-4.

Therefore, pH paper will register this slight acidity, turning a shade of orange or yellow, indicating a pH below 7.