The colour shift results from pH affecting the phenolic groups on the carminic acid molecule.
Extracting carminic acid in the presence of aluminum or calcium ions results in carmine. "Carmine" is the common term for insoluble cochineal-based pigments. These hues follow the same shifts as carminic acid but will precipitate at low pH before becoming orange. Acid-proofing results from a hydroxyl group being replaced by an amino group and results in red to violet hues in low pH applications where carminic acid would otherwise be orange.