Ab initio calculations have been carried out to unravel the relationships between the structure and the first hyperpolarizability in flavylium derivatives, with the aim to design efficient second-order nonlinear optical (NLO) switching compounds. Large contrasts of the first hyperpolarizability have been obtained along the pH-controlled and photoinduced transformations for specific combinations of chemical substituents in the 4'- and 7-positions, which demonstrates that these multistate systems should behave as highly efficient molecular NLO switches.