Display omitted •The acidochromism of C3PY in different solvents and solid-state were investigated.•The dielectric constant and basicity of solvent influence the acidochromism behavior.•A general mechanism for acidochromism was proposed.•The application of C3PY as acid sensor was explored. Molecules with stimulus-response properties have always been a research hotspot. However, the acidochromism of enone dyes was rarely studied. In this work, the solvatochromism of an N-methyl pyrrole-based enone dye (C3PY) in nine organic solvents was firstly investigated by Kamlet–Taft equations. Then, with trifluoroacetic acid (TFA) as an example, the acidochromism of C3PY was investigated in nine solvents and solid state. Different acidochromism behaviors were observed and analyzed, which were discussed in terms of the solvent properties such as dielectric constant and basicity. A general mechanism was proposed and was supported by the 1H NMR acid titration experiments. Finally, the reversibility of acidochromism was evaluated and the application of C3PY as a TFA sensor was explored.