1. A system comprising: an electron microscope configured to generate an image of a nanoparticle sample containing a plurality of nanoparticles, by irradiating the nanoparticle sample with electron beams; wherein the nanoparticles comprise a plurality of spectrally distinct optical emitters configured to generate CL (cathodoluminescent) light at respective different color channels in response to the electron beams from the electron microscope; and a CL detecting imaging system configured to detect the CL light from the nanoparticles and to generate multi-color CL images of the nanoparticle sample; wherein the nanoparticles comprise nanodiamonds that contain NV (nitrogen-vacancy) color centers, and that are configured to produce red cathodoluminescent light at a wavelength of about 620 nm.