Indicators are substances that change colour in response to a change in the acidity or alkalinity of a solution. The most common indicators are:

* Litmus: Litmus is a natural dye that is extracted from lichens. It turns red in acidic solutions and blue in alkaline solutions.

* Phenolphthalein: Phenolphthalein is a synthetic indicator that is colourless in acidic solutions and turns pink in alkaline solutions.

* Methyl orange: Methyl orange is a synthetic indicator that is red in acidic solutions and yellow in alkaline solutions.

* Bromothymol blue: Bromothymol blue is a synthetic indicator that is yellow in acidic solutions, green in neutral solutions, and blue in alkaline solutions.

These are just a few examples of the many indicators that are available. Each indicator has its own unique colour change, which makes it useful for different applications.