The other thing is that full-color, textured icons help a great deal with immediate recognition as well. It's easy when the "open" icon is a yellow folder.
Sadly, the move to flat design with monochrome icon outlines destroyed that entirely. I'm still waiting for full-color icons to come back in style again. They can still be flat, but being filled with color (not just colored outlines) helps immensely with fast recognition.
Color helps for some people but when making an accessible design you have to account for color blindness and other vision issues. Which means the icon needs enough contrast and can’t rely on just color to convey meaning, unless the icon is paired with a text label that also conveys the meaning the color was trying to convey.
So while I agree that color iconography can be extremely helpful for the average user, unless it is supplemented by other techniques it can make a product completely unusable for a subset of users
For accessibility reasons, you shouldn’t rely on only the color, but that doesn’t mean that color isn’t a very useful property to apply.
The commenter said that adding distinct colors to icons help users tell them apart.