Comment by jfengel

Note that the "rule of 3 colors" is about three hues. The full design will incorporate variations of those colors: brighter and darker, more and less saturated. Your brain actually thinks of these as "the same color", though HTML and CSS don't.

It's a design principle which is safe: it's not going to clash in an ugly way, or make combinations that are hard to read. They can be used (in conjunction with other design principles) to draw the attention where it's needed, and have the eye flow through the page without distraction.

"Safe" and "boring" are two sides of the same coin. I personally don't mind boring, and easily get aggravated by choices that distract. A truly great design breaks the safe principles, though usually at a high cost. (Usually, having tried and rejected many combinations.)