The important thing is to consider the function of a flag. A flag is a flexible, piece of fabric that is suspended in the air to use as a form of identification or communication at at distance.
So, a flag needs to be recognizable, and it needs to be so when suspended in the air, potentially wrinkled upon itself to some degree, and at a distance where fine detail is not going to be visible. It might also be seen from either side.
The "Rule of Tincture" from European heraldry is a rule of thumb for making high visibility colour combinations and could be generalized to "contrast light with dark". It's not about following the rules for the rules sake, it's about making the flag effective.
A few similar "rules" would be to try to have boundaries between contrasting colours extending to the edges of the flag. This helps make the flag recognizable even if it's hanging limp without much wind.
Don't use fine details to distinguish your flag. They won't be visible at a distance or if the flag is moving or limp
Don't just drop a symbol like a seal or coat of arms in the middle of a solid coloured field (US State Flag Syndrome). Derivatives of red and blue ensigns (Australia, New Zealand, Ontario, Manitoba, etc) are another bad example. An alternative is to do a heraldic banner where you take your coat of arms and expand the shield into a flag. (The flag of British Columbia was created this way for instance)
Don't make the back and front of the flag different. A few flags have distinct back and front designs and this is a horrible idea. The point of a flag is to be recognized so having it be different depending on the side you look at is counterproductive. It's also difficult to make a flag that's different on opposite sides without making it either "bleed through from one side to the other" or be too heavy to fly properly.
The flag should work when mirrored. The back of the flag is actually a mirror image of the front. The US flag has the blue canton with stars at the left on the front, and at the right on the back. Think in terms of toward/away from the flag pole instead.
Don't include text. Text is fine detail that doesn't work when mirrored.
Consider other flags being used and aim to be recognizably different. Don't be like Australia and New Zealand (Which one has the extra star and which has the stars trimmed in red?), Ontario and Manitoba (Does the little tiny coat of arms have a sprig of maple leaves or a bison?), The Netherlands and Luxembourg (What shade of blue is that?), or most of the US states (So it's some sort of circular seal on a blue background)
Of course, that all depends on whether you are trying to come up with good flags. There are obviously a lot of bad flags in real life, so you might want to make intentionally bad flags. Modern flags tend to be fairly symbolic so they can survive being poorly designed. Historically their function was more important so simple bold recognizable designs would have been preferred.