Use of multiple proteins tagged with different genetically encoded fluorescent proteins. Shcherbakova et al. report how they used a gap in the fluorophore spectrum by using random mutagenesis and directed evolution to engineer a bright monomeric orange fluorescent protein with a large Stokes shift, LSSmOrange (excitation/emission 437/572nm). 

They demonstrated the utility of LSSmOrange by imaging four proteins simultaneously using fluorescence cross-correlation spectroscopy in vitro.   Nature Methods 9, 6, June 2012. Shcherbakova, DM, et al. J. Am Chem Soc. Advance online publication 24 April 2012.