Optically switchable dual-color fluorescent nanoparticles that incorporate two classes of dyes into the polymeric chains have been synthesized using an emulsion polymerization method. The nanoparticles consist of an organic photoisomerizable dye, spiropyran, as an optically responsive component and another fluorescent dye, perylene diimide, as a high-energy emitter. Under UV irradiation, the colorless spiropyran undergoes photoisomerization to yield merocyanine, which absorbs at 588 nm and fluoresces strongly at 670 nm. The absorption band of the merocyanine matches well the fluorescence bands of perylene diimide (at 535 and 575 nm), and therefore fluorescence resonance energy transfer (FRET) converts the high-energy green emission of the perylene into low-energy red emission when the merocyanine form is present. Upon exposure to alternating UV (< 400 nm) and visible (> 420 nm) light, the nanoparticles cycle between red and green fluorescence as the spiro and mero forms of the optically responsive component interconvert. The relative photoluminescent intensities of the green-fluorescence perylene diimide and red-fluorescence merocyanines can be controlled by varying the feed ratio of perylene diimide and spiropyran monomer during polymerization. These dual-color fluorescent nanoparticles were developed as potential new tools for biomedical applications and live-cell imaging. When delivered into HEK-293 cells, they display either red or green fluorescence, depending upon the wavelength of light to which the cells are highlighted.