Free to view or download, the Guiding Principles of Flag Design (Flag Institute/ North American Vexillological Association, 2014) are full of best-practice advice for every flag designer.
Covering terminology, basics, colour, structure, devices and symbolism, the guide advises on elements that tend to work well or do not.
But it does not cover aesthetic appeal, and remember that you can almost always find a successful rule-breaker.
The flag of South Africa and the Pride Flag, for example, both use multiple colours to great effect.
So enjoy your designing, have confidence in your flag, and do whatever you feel works best.
