Nor does it appear to be a photograph. It looks very much like a 3D rendering with false coloring.
In this experiment, the electrons interact with the photons to produce quantum energy packets, which the microscope detects. What is the colour of the quantum energy packets? The question has no meaning, because they aren't light. Since the colour is arbitrary, they can choose to colour it in any way they find useful.
Having said that, I also don't understand the "photograph".
Actually, we do, so long as that energy is in the form of photons [1].
[1] http://www.nature.com/news/people-can-sense-single-photons-1...
Of a particular range of frequencies