This is nearly as bad as grey/monochrome icons, which is the way Google chose to ruin the GMail interface years back. In both cases you're completely discarding one great way to let users quickly and easily distinguish between different icons: color.

Apple did the same thing with the iTunes interface (colors -> grey) even longer ago, and I just don't get it. Different colors are objectively good for the UX. It's information that gets instantly processed by your brain.