We demonstrated all-optical modulation with a nonlinear medium, i.e., indigo carmine, an aromatic conjugated structure with delocalized π-electrons, using non-high power continuous wave light for pump and probe of different visible wavelengths. Pump-induced probe transmission increase occurred through absorption saturation of probe light by pump-induced linear and nonlinear absorption including two-color excited-state absorption (ESA). The two-color ESA occurred only when both pump light and probe light co-propagated through a medium, leading to nearly pump power-independent increase in probe transmission for appropriately chosen wavelengths of pump and probe light, given the optical transition structure of electronic energy levels in the medium.