Colours help to differentiate between icons. It's much easier to differentiate between and identify two differently coloured icons at a glance, than two monochrome icons.
To me it looks like they've put more effort into differentiating the shapes now that the icons aren't colored. Personally I think the increased contrast makes up for it -- the shapes in the monochrome icons are much more visible to me. I'd love to see their usability studies though. Perhaps you are right.
