The generator generates flags as svg images and display them on the generator page. The flags can be saved with the link provided.
The generator first chooses a set of color couples to use (a dark color and a light color). Then it chooses the main division of the flag (plain color, bicolor or tricolor, horizontal or vertical, ...). It terminates by choosing an overlay that adds elements like stars or crosses on the flag. The choice of overlay depends on the division.
The generator uses a rule when combining colors: light colors are placed above dark color (and vice-versa). This is inspired by a heraldic rule regarding how colors are combined: no metal (light color) on metal nor tincture (dark color) on tincture. The generator draws inspiration from heraldry and there are indeed many heraldic references inside the source code.
Why not providing a flag editor ?
Maybe as a future feature. I need to design an intermediary format that describes a flag before being able to provide an editor. Currently, the generator directly outputs geometric shapes when generating the flag.
There exists websites allowing you to design flags. For instance Scrontch's Flag Designer.
Licence of the generated flags
Since I don't care at all about what you do with the generated flags, the generated flags are released under the Creative Commons CC0 1.0 Universal (CC0 1.0) Public Domain Dedication. You can still put a link to the generator or credit me if you want.