A person who studies flags is called a vexillologist, and a person who designs flags is a vexillographer. Vexillologists and vexillographers have common standards and “rules” as to what makes a flag design appealing or not.
Here are the North American Vexillological Association’s five principles to a good flag design:
The flag should be so simple that a child can draw it from memory.
The flag's images, colors, or patterns should relate to what it symbolizes.
Limit the number of colors on the flag to three which contrast well and come from the standard color set.
Never use writing of any kind or an organization's seal.
Avoid duplicating other flags, but use similarities to show connections.
Dominican Republic
Bangladesh
Alaska
Dominica
Bey of Tunisia
Virginia
The many colors on South Africa's flag each have deep symbolic and historical meanings.
The "CALIFORNIA REPUBLIC" text on California's flag makes this design memorable and iconic.