Answer is "an indicator".
Indicators change its color according to the pH of the solution. They shows different colors when the medium is basic and acidic.
Indicator pH range Acidic color Basic color
Methyl orange 3.1 - 4.4 red yellow
Phenolphthalein 8.2 - 10.0 colorless pink
Bromothymol blue 6.0 - 7.6 yellow blue
