Shorthand on Colors

Red should look like fire rather than venal blood.

Green should be lightest and brightest shade, so it actually looks fresh and not deep green like deep jungle.

Blue should be either very dark or very bright and intense.

Yellow should be as bright and intense as rails on public transport. If it's too pale, it looks like it's just some old yellowed white.

Also, dark colors should be close to bright one, not two bright together or two dark together.

I think I mentioned before about light next to dark contrast principle. That is a general design principle. Take flag of France for example. Very Dark Blue and rather dark red is divided by bright white. That is why it looks nice.

In heraldry they simplified that further with rules of tincture: no color on color and no metal on metal. They call yellow and white metals because these colors are bright. At the same time, they call red, blue, green, purple and black colors because they are darker.

If you use only light colors, it will look childish, if you use only dark colors, it will look old and dilapidated. However proper contrast will allow the flag look good.