Bromophenol blue Indicator is recommended as a pH indicator, a colour marker and as a dye. Bromphenol blue is an acid-base indicator structurally related to phenolphthalein (a popular indicator). It changes colour from yellow at pH 3.0 to blue at pH 4.6; this reaction is reversible. Bromophenol blue is also used as a color marker to monitor the process of agarose and polyacrylamide gel electrophoresis and also used as a dye.