https://flagpoles.co.uk/flag-design-principles/

The article argues that a well-designed flag should combine simplicity, meaning and clarity so it works well in real contexts.  It says a good flag must be easy to recognize even from a distance or when fluttering which means avoiding overly complex detail. There should be only a few colors and should be chosen for symbolic or cultural significance, and contrast enough to stand out. Symbols or shapes (not text or seals) should reflect the identity or values of what the flag represents. Harrison Flagpoles Finally, the design should remain effective at different sizes and contexts (from a tall flagpole to a small badge), and be distinct so it’s not easily confused with other flags.