Via Gizmodo:

To create the colorized scans, the researchers fitted a special detector on a conventional electron microscope. The researchers then selectively “painted” structures such as proteins, membranes, and cells with various “rare earth” metals, including lanthanum, cerium and praseodymium in the form of a chemical solution. When these specimens were scanned by the modified microscope, the stream of electrons lost by the metallic elements were interpreted as color.

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