Is it possible designers have some principles that don't necessarily match real life usability? Only 3 colors is incredibly boring to me.
I see that hacker news also only has 3 colors, it's fine in this case because there's not much reason to have more. That doesn't apply to, say e.g., photoshop (and gimp following this too now for unknown reasons) only having 2-color icons, while multicolor ones are faster to distinguish visually
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.)
If it helps, you can enable the old style icons in preferences.