It is interesting. I will need to check sources (Samir Zeki's vision book is amazing), but color detection happens way sooner in the v2/v3 region than symbol interpretation which is a higher level function. This is why stop signs are red, traffic lights are not icons (even if distance wasn't a concern, icon based traffic lights would take too long for human vision system to process). Evolutionarily, certain colors such as red indicate threat, injury, decay or food (blood) and we're hardwired to detect it effortlessly and within 100 ms or so. Before the interpretation (v4/v5/limbic system) happens. Baring color blindness concerns, monochrome iconography is objectively worse in every way except for the reasons I'll discuss below.
If you study mission critical systems, even a fork lift, colors are everywhere. EMO button is red. CNC control panels have lots of colors.
The reason why we use flat symbols (recylcing symbol on a milk jug, hazard labels on chemicals, bathroom symbols and airport signs, and road signs) is a practical consideration about printability and ease of application (single printing ink, stencils ), color fastness in the sun, etc. It's not for the reasons you're alluding to, although some of those concerns are orthogonally valid - layout should be logical and flow should be intuitive. Color icons are far superior, if someone can publish a scientific study, I would bet on it with real money. They might be ugly, not against brand/identity/etc. but I am strictly speaking of their utility.
But you can do colors with icon fonts! In fact, one of their benefits is that you can do context specific colors. So only have things be red or green when it's most meaningful.
I think the problem is in having all icons be bright and meaningful all the time - too much visual information can be worse than too little.
I guess I don't understand this critique because with a monochrome iconset, I can style them to have the semantic color that's relevant at the time.
A dangerous or destructive operation can be made to be red, to indicate the danger of the action.
A primary action that I expect the user to do can be made a primary action color.
If the icons are _built_ with color, then I can't change the color based on the semantics of how the icon is used. If you give me a monochrome icon that I can style, then I can match the color to the placement and functionality of the icon.
So, yes, _in the application_ the icons shouldn't be monochrome. But in the _iconset_ I would generally prefer they be.
If you study mission critical systems, even a fork lift, colors are everywhere. EMO button is red. CNC control panels have lots of colors.
The reason why we use flat symbols (recylcing symbol on a milk jug, hazard labels on chemicals, bathroom symbols and airport signs, and road signs) is a practical consideration about printability and ease of application (single printing ink, stencils ), color fastness in the sun, etc. It's not for the reasons you're alluding to, although some of those concerns are orthogonally valid - layout should be logical and flow should be intuitive. Color icons are far superior, if someone can publish a scientific study, I would bet on it with real money. They might be ugly, not against brand/identity/etc. but I am strictly speaking of their utility.