Let Eng in me comment on color question: it is true that one needs certain size to reflect color. That is because less than half wavelength size tend to pass the wave over the object.
But light has a large spectrum. In your comment you meant visible light.
Any material can absorb one wavelength and emit another. That is seen in dispersions liquids. That is called as Tindal effect. This materials can absorb a shorter wavelength and emit larger one.
That is, theoretically, size of the object can not decide color completely.
Usually, the colors we see are reflection colors. Those are affected by the size. But if we take a single atom and allow it to emit a photon, it will emit a photon based on how many energy levels outer electron drops and what was the eV value of that drop.
The atom and molecular structures decide the energy levels outer electrons will drop or absorb. That will decide what wavelengths will be seen by the outside world. The constant which decides the relation bet energy level and frequency/wavelength is Planks constant.
That is why the colors of stars can tell you temperature. Higher the temp, higher the frequency because the outer electrons are at higher eV level. When they drop to normal level the frequency they emit is higher ( low is red, high is violet). This decides the color too.
So, your were generally right that for reflected light, size of the object is important. But that is not the complete story.
You can inject or mutate the virus to make material which can emit light if excited at a temperature. Then one can make viruses emit visible colors.