We describe a multi-color microfluidic dye laser operating in whispering gallery mode based on a train of alternating droplets containing solutions of different dyes; this laser is capable of switching the wavelength of its emission between 580 nm and 680 nm at frequencies up to 3.6 kHz - the fastest among all dye lasers reported; it has potential applications in on-chip spectroscopy and flow cytometry. © 2009 The Royal Society of Chemistry.