Substances that have distinctly different colors in acidic and basic media are called what?

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Substances that show different colors in acidic versus basic solutions are pH indicators. They exist in two forms depending on the proton concentration: one form is favored in acidic conditions and the other in basic conditions. Each form absorbs light differently, giving a distinct color. The switch between colors happens over a specific pH range, which makes these indicators useful for judging how acidic or basic a solution is or for spotting the endpoint in a titration. They aren’t catalysts, buffers, or solvents; catalysts speed up reactions, buffers resist pH changes, and solvents dissolve solutes. Examples include litmus, phenolphthalein, and bromothymol blue, which visibly change color as the environment shifts from acidic to basic.