0.08 0.10 0.12 0. Of those three complexes, only the mixed red/green species displays a FRET signal and consequently the intensity of the red fluorescence signal halves during ligand mixing. Upon 505 nm excitation the green/green complex displays green emission, while the red/red complex shows negligible red fluorescence (originating only from cross-talk). The slow decrease in the concentration of the FRET complex initially formed, leads to a decrease in the red signal and an increase in the green signal.