Bromothymol blue turns different colors in different solutions, depending on the pH of the mixture. It serves as a heavy acid in the solution. This is bluish-green in neutral water - the neutral form's deprotonation results in a highly conjugated structure accounting for the color difference. And, the greenish color in the neutral solution is responsible for a deprotonation process intermediate. This deprotonated form of the compound has its peak absorption wavelength fixed at 427 nm, therefore transmitting yellow light in all acidic solutions.