CuBDC MOFs can realize the sensitive and accurate colorimetric detection of S²⁻ in water based on two complementary mechanisms. As reactive metal centers in MOFs for S²⁻ responding, Cu²⁺ ions can be taken out from the MOF nodes by S²⁻, inducing the generation of sandy brown CuS. The ratio of blue CuBDC MOFs and sandy brown CuS in an aqueous solution is varied with the amount of S²⁻ added, leading to visual changes in the solution color according to the principle of pigment mixing. The presence of S²⁻ can be indirectly confirmed and quantitatively monitored through the UV-Vis spectrophotometry since the capture of Cu²⁺ by S²⁻ can induce the recovery of uncoordinated carboxyl groups in CuBDC MOFs accompanied by the increase in absorbance intensity. Moreover, CuBDC MOFs can be immobilized in polyvinyl alcohol (PVA) gels to form test strips that can realize S²⁻ detection in water samples with a linear range of 10–100 μM. Additionally, the PVA gels can be dried, accompanied by arbitrary bending deformations for the portable detection of S²⁻ in water. The design strategy based on two complementary mechanisms promises to provide an accurate, sensitive and portable colorimetric assay for S²⁻ detection in water.