I went on a tour of a miso factory today and learned about how it's made!
What surprised me the most was that shiro (white) miso and aka (red) miso are both the same mix of soybeans, salt, and rice malt but fermented for different periods of time. As the miso ferments for longer, its color becomes darker while its flavor becomes milder and more complex. Beyond 3 years of fermentation, you get diminishing returns as its flavor becomes too acidic.
After the tour, we got to sample some of the naturally fermented 3 years old miso, and it was easily the best I've ever had. Most miso you can buy in a grocery store is created through forced fermentation over a few months, so if you ever get a chance to try naturally aged miso I would highly recommend!