Some plants have flowers that change colour when you add a chemical called lime to the soil.
Some people add lime to help the plants grow better.
The dye in the petals which gives the flower its colour is showing, or indicating, how healthy the plant is. The back of a spoon can be used. Try any darkly coloured petals or juice from vegetables and fruit. Beetroot juice, red cabbage, blueberrires and even tea works. Take careful note of the colour of your indicator.