Bromothymol blue is a pH sensitive dye that has been used as an indicator in many laboratory reactions. from MeSH. Bromthymol Blue is a dye used as an indicator in determining pH. Bromthymol blue is a weak acid. It can be in acid or base form, depending on the pH of the solution. This reagent is yellow in acidic solutions, blue in basic solutions and green in neutral solution. Pharmacology from NCIt.