While most of the time bacteria are invisible to the naked eye, that does not stop some species from producing different colorful pigments. A non-exhaustive list of bacteria and what colors they are can be found here.
These colors aren't just produced to make us humans happy (although they certainly can do that), they can serve a purpose to the bacteria.
Sometimes when bacteria produce a pigment, it is simply because the gene to make that pigment is close to a gene for a more important protein. This could be a gene for a red pigment really close to the gene for a compound that lets the bacteria break down starch.
Other times its the pigment itself that is useful. Chromobacterium violaceum produces a purple pigment that has shown to have antifungal properties. That means this pigment can help this bacteria out-compete fungi that may be using the same resources as it.
Sometimes within the same genus, different bacteria species can be a different color from one another. Take for example Psuedomonas aeruginosa and Psuedomonas putida. P. aeruginosa produces a blue color while p. putida does not produce such a color.
Some scientist chose to get creative with this and make art. The American Society for Microbiologists hosts an art contest each year. To learn more about agar art, such as the picture at the very top of this article, visit the ASM website! Maybe you can even participate in it yourself!
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