My philosophy on flags begins with this question: Why have flags? My answer is that a flag is meant to be seen, especially on the battlefield, to show the identity of its bearer. In order to do this, a good flag is at once simple and distinct. It should be able to tell a story in miniature without even saying a word.
Bad flags, like the old Libyan green flag or the various blue US state flags, are not distinct or simple, respectively. Good flags though, like New Mexico’s or Jamaica’s, are both.
Personally, I don't mind flags that indulge in complex designs or many colors as long as the large elements are recognizable from a distance. The flags of New York City, Dominica, Belarus, Austria-Hungary, Comoros, Joseon, Qing, Yukon, Tibet, Yellowknife, the Buddhist religion, the CAR, Mexico, or the Bolivian Wiphala are good examples. Mozambique has one of my favorite flags in the world, and Bhutan, Brunei, and Venice are really complicated but really cool
and distinctive from a distance, I would argue. They may be complex or colorful, but they are still very recognizable. I also admit that I really like the flag of Turkmenistan.
There are many different aesthetic philosophies, color theories, symbolic systems and values in flags as well. For example, the plain green flag of the Jamahiriya was the flag used by the Fatimid Caliphate, directly calling up the image of an Islamic golden age and a pan-North African idea. It's both meaningful and recognizable in the same way as the plain black flag of Anarchism.