We present multicolor imaging of cells in a high-speed flow with molecular vibrational contrast based on stimulated Raman scattering (SRS). By using a newly developed wavelength-switchable laser, four-color SRS imaging is accomplished with an unprecedented pixel rate of 4.75 Mpixels/s. This SRS imaging system was integrated with a microfluidic chip equipped with an acoustophoretic focuser to realize four-color SRS imaging at a flow speed of 2 cm/s. In this paper, we introduce the principles of SRS imaging, SRS flow cytometer, and wavelength-switched lasers, and then demonstrate multicolor SRS imaging of polymer beads and microalgal cells.