If presented with a design full of these errors, and presented with an otherwise-identical design that has these errors corrected side-by-side, you will almost certainly prefer the one with the errors corrected, even if you can't put your finger on why.

I'm not very good at design, I'm especially terrible with colors, but if you study these mechanics a little you can significantly improve the look of your websites just by mechanically following these rules. It does work; a startup I worked at created a report of vulnerabilities on your website and by blindly following the principles of good design actually produced a very good looking report, better than the competition that could afford designers (but apparently didn't). It was a subtle thing, but putting our report next to the competition's report really made it look as if we were the ones with years of experience and knew what we were talking about, instead of the upstart newcomer. It failed, ultimately, but not because of our report format :)