Bromthymol blue is a pH indicator that changes color depending on the pH of the solution it is in. At a pH below 6.0, bromthymol blue is yellow. At a pH between 6.0 and 7.6, bromthymol blue is green. At a pH above 7.6, bromthymol blue is blue.

When bromthymol blue is exposed to CO2, the CO2 dissolves in the water and forms carbonic acid. This causes the pH of the solution to decrease, which in turn causes the bromthymol blue to change color from blue to green or yellow.

The exact color change that occurs depends on the concentration of CO2 in the solution. At low concentrations of CO2, the solution will turn green. At higher concentrations of CO2, the solution will turn yellow.

The color change of bromthymol blue can be used to monitor the concentration of CO2 in a solution. This is useful in a variety of applications, such as monitoring the air quality in a room or the fermentation process of beer or wine.