Flags, why do so many countries choose boring ones?

[Jacktannery]

I'd also advise you to lighten up the colours somewhat - your shades are a little bit dull. Here is an example of red and green as complementary (these shades are directly opposite each other on the colour wheel, and therefore look a little more vibrant).

EDIT: not suggesting you actually go with something this brash - just saying.

red-green.png

[EricPoehlsen]

Some aspects have been already mentioned. Flags need to be produced cheap in numbers and be easily recognized over a distance. The colors in heraldy were traditionally limited:
'Colors'
██black ██red ██blue ██green and ██purple
'Metals'
██silver and ██gold

The combination should be color to metal. So heraldic correct there are 7 singlecolor flags, 20 twocolor flags and 70 threecolor flags (+70 if turned 90°)
some colors are rare. Purple and black were hard to manufacture in the past, so they are not too widespread.

I just found the flag design principles published by the North American Vexillological Association

[ravells]

Great find, Eric! I will repost the link in the resources section - have some rep!