I was in a senior position at Microsoft back at the time early versions of Office, Windows 95 and 98 were being developed. In fact a number of groups including the Visual Interface Design group reported to me. That's the group that designed the general visual appearance and also icons in the UI for both Office and Windows. (They were not designing the actual user interaction, just the visual appearance.)

At that time the visual designers were strongly urging that all icons be greyscale because they said color was "distracting". I overruled them and insisted the icons have color because it was better for overall usability.

Now the whole industry seems to have come under the influence of the visual designers favoring visual appearance over usability. Much less attention seems to being given to real overall usability.