We report the synthesis of two new three-dimensional coordination polymers (CPs) based on Co(II), adenine, and aromatic tetracarboxylate linkers. Adenine exhibits bidentate binding modes in both CPs, coordinating through the N3 and N9 sites in a first compact CP and through the more rare N3 and N7 sites in a second open, flexible, and H2O-responsive CP. These differences together with an analysis of the extended coordination structures made of adenine reported in the Cambridge Structural Database illustrate the rich coordination versatility of adenine as a building block for CPs. Although the latter CP is nonporous to N2 or CO2, it shows a reversible and detectable color change from pink to purple, and vice versa, upon hydration and dehydration, respectively. ; I.B. thanks SENESCYT-Ecuador for a Ph.D. fellowship. I.I. and K.C.S. are grateful to MINECO, for a Ramon y Cajal grant, and to the EU, for a Marie Curie Fellowship (300390 NanoBioMOFs FP7-PEOPLE-2011-IEF), respectively. ICN2 acknowledges support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant SEV-2013-0295. ; Peer Reviewed