FWIW, I first noticed the "use monochrome icons to reduce distraction" approach in Windows 7, when it was applied to the default system tray icons. I think that predates the Apple trend (but I don't follow Apple closely enough to know).

I suspect these kinds of things tend to just be general "design" trends though - I bet the people designing these interfaces probably talk to each other, subscribe to the same magazines, read the same articles, follow the same non-software industrial designers and artists etc.