This work provides a novel approach for the instant detection of CN− anions based on chromogenic 6-[(E)-(4-nitrophenyl)diazenyl]-1′,3,3′,4-tetrahydrospiro[chromene-2,2′-indole] derivatives. New colorimetric detectors were synthesized and characterized. These compounds exhibited a substantial color change from orange to magenta and blue when treated with cyanide ions in a CH3CN solution buffered with sodium phosphate and demonstrated high selectivity to CN− anions. Common anions were tested, and they did not interfere with cyanide detection, except for carbonates and hydrosulfites. The simple preparation of a molecular sensor and the easily noticeable color change makes this a practical system for the monitoring of CN− ions. This color change is explained by nucleophilic substitution of the pyrane ring oxygen atom at the indoline C-2 atom by the cyanide anion. This generates the appearance of intensively colored 4-(4-nitrophenylazo)phenolate chromophore and allows for determining very low levels of CN− anion.