The two forms of the indicator molecule have noticeably different colors. For example, bromocresol green has a yellow HIn form and a blue In form. In, the solution looks bright green. Adding a drop of base converts HIn to In, and the solution becomes more blue. To understand the answer, we'll have to know a little about how color and molecular structure are related.
Light delivers energy in little packets called photons .
Different colors of light pack different amounts of energy in their photons. For example, photons of violet light have almost double the energy of those of red light. Different molecules absorb different colors of light, depending on their electronic structure. All materials absorb photons of some energy. But only substances that absorb photons of visible light will have color.