A popular cafe drink is made with blue flowers (Unchun) that grow wild in open spaces in Bangkok and elsewhere. Extract the blue dye by boiling the petals in water.
The dye changes colour with the addition of a few drops of vinegar and a few drops of caustic soda solution. Lime juice also changes the colours.
Unchun dye loses its colour after two weeks in water but is more stable in acidic solutions, which remain unaltered over three weeks. A stable yellow dye remains after several hours in NaOH solution. The yellow dye has no colour in acid.