1. A method comprising: irradiating with a scanning electron beam a nanoparticle sample containing a plurality of nanoparticles, so as to cause emission of CL (cathodoluminescent) photons from the nanoparticle sample; wherein the nanoparticles comprise a plurality of spectrally distinct optical emitters configured to generate CL light at respective different color channels; and detecting the CL light from the nanoparticles to generate multi-color NP (nanoparticle)-CL images of the nanoparticle sample.