Diagram showing a possible scenario for barred and unbarred discs across the galaxy CMD. Galaxies start their evolution as star-forming blue unbarred galaxies and eventually may or may not develop a bar, depending on internal conditions. When these galaxies enter into the green valley phase, unbarred galaxies remain unbarred whereas many bars in barred galaxies are destroyed (e.g. via interactions). Unbarred disc galaxies can cross slowly or rapidly along the green valley (represented by narrow and wide green arrows, respectively) while barred galaxies only experience slow star formation quenching. When disc galaxies reach the red sequence, barred ones remain barred whereas unbarred ones can develop a new bar.