Why do microorganisms appear green?

Answer 1

It has to do with how the organism is dyed, and the lighting. Without the dye, the organism would blend in and wouldn't stand out.

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Not all microorganisms in their natural habitats are green. The ones that are green, such as cyanobacteria, green algar, etc. are green because they conduct photosynthesis with a mechanism similar to plants.

(Note for previous answerer: microorganisms are very often stained in a laboratory setting to make them more visible, however green stains are rare because they would make stained organisms look confusingly similar to photosynthetic microorganisms. There are, however, fluorescent stains that fluoresce green when the stained organism is exposed to UV light, this would not be confusing since photosynthetic organisms no longer look green under UV light.)